Aboria: to enter it, you need a Chronomancer. After Brilhasti, I have 2 mages that are Archmage 3 now.
I assume that I cannot classchange back to AM sometime later. does this mean I have to give up now one of my mages on Archmage Level 3 only, so to become Chronomancer ?

If you want to continue the game, yes. Chronomancers are required to go to the other dimensions. You don't need to worry about them only being a low level Archmage, though; when your Archmage gets turned into a Chronomancer, they lose all of the spells that they previous learned, so even if your Archmage could cast MAMA, they would lose that ability when they turned in tho a Chronomancer.

At least in classic versions (I haven't checked the remaster), this technically isn't entirely true; if you have any of the miscellaneous magic spells, you retain them after the change. Of course, since the first such spell isn't obtainable before Arboria (which requires a Chronomancer to reach), and few players will consider using multiple Chronomancers, chances are this dosen't actually matter in practice.

By the way, the requirement to change into a Chronomancer is different in the remaster; originally, it required learning level 7 spells of 3 types, but now it instead requires that you get level 3 Archamge spells (but you don't need spells past 3rd level for any other types). This means that, if you transferred a BT2 party with a spellcaster who rushed Archmage (an interesting strategy, even if not ideal in the long run) to BT3, you can choose to make that character your Chronomancer. Supposedly beating Brilhasti is also required to change into a Chronomancer, but I have not verified that.

Also, I should point out that you will want at least one Archmage who knows all the spells in BT3 (if you didn't skip any spell levels, this will happen immediately after Brilhasti), as you will need some of the spells to solve puzzles later; furthermore, many of the Archmage spells are extremely useful and have no Chronomancer counterparts. (APAR and BASP are the two that come to mind here.)

Fortunately, if you've already played through the starter dungeon, it's rather easy to beat it again with a transferred party; just teleport to the portal on level 3, go down, make your way through level 4 (it helps to have a Nospin Ring, but otherwise the light song will mitigate the spinner issue), and just spam MAMA against Brilhasti.

By the way, one interesting note:
* In the remaster, if you defeat Brilhasti, go back to the refugee camp without getting the reward, switch to an entirely different party (no shared characters), and go back, your new party will get the reward. This does not happen in classic versions, as quest event flags are stored for individual characters rather than the party as a whole.

And another couple of facts (rot13 encoded because they contain a major late-game spoiler):
* Va pynffvp irefvbaf, vs gur byq zna vf qrnq naq lbh erghea jvgu n serfu cnegl, gur byq zna jvyy or nyvir naq jryy.
* Va gur Pbzzbqber 64 naq QBF irefvbaf (cebonoyl Nzvtn, ohg abg Nccyr 2 V oryvrir), vs lbhe cnegl pbagnvaf bayl zbafgref be vyyhfvbaf (be vf rzcgl, juvpu vf npghnyyl cbffvoyr), gur byq zna jvyy abg or gurer vs lbh gel gb ivfvg uvz. V oryvrir guvf unccrarq nf n erfhyg bs n svk gb na rkcybvg ba gur Nccyr 2 irefvba gung jbhyq nyybj lbh gb ercrngrqyl trg n 600,000 KC njneq whfg ol fhzzbavat n zbafgre, trggvat gur erjneq, qvfzvffvat gur zbafgre, naq ercrngvat (V tbg na Nepuzntr cnfg yriry 200 onpx va gur qnl guvf jnl).

Why ever not? A large part of the game takes place before you learn NUKE, and Fatal Fist is dramatically better than Mangar's Mallet. Or, really, anything Archmages can cast that isn't NUKE or Divine Intervention. Yeah, Archmages get the cool utility spells, but you don't need more than one Archmage for those. If you're transferring from BT2, you almost certainly have all your Archmages leveled to the point where they level really slowly, so your Chronomancer will get tons more SP as well, for more dungeoning endurance.

So in general, I'd submit that not only can it be done, but it's the objectively most effective way to build your party if you have enough casters to get two Chronomancers and still have the required Archmage.

That said, I'm big on casters in my party. I also make 2 Geomancers when I get the chance. The only non-casters are the required Rogue and the Bard who, while not strictly mandatory, is super-useful.

(This post applies to the remaster; classic versions handle the Brilhasti bonus in a way that is strategically different.)

However, if you're transferring from BT2, there is a chance you'll already have MAMA, which is stronger than anything (non-Misc) a Chronomancer can cast other than FAFI. If you're playing with legacy XP (which is how I prefer to play the remaster), and you don't fight that many fights before Tenebrosia, you may find that you get NUKE before FAFI, or that it will at least be a while before you get FAFI, in which case your best offensive spells are MAMA (from Archmage) and DIVA (Misc., and rather expensive) I hope you have a Conjurstaff (or equivalent) and enough Harmonic Gems.

(In the classic versions, if you kill Brilhasti with an Archmage or Chronomancer in your party, that character will get enough XP to reach level 35, allowing you to get FAFI early.)

There's also the fact that, in the remaster, an Archmage transferred from BT2 can equip the Death Dagger, but a Chronomancer can not; this might matter if you are big enough on casters to put one in the front ranks. I think I may actually try this. (Note that the Death Dagger will not drop in BT3, not even in the remaster, as far as I can tell; nor will the other BT1-only items.) (Again, tis post pertains to the remaster.)

In the remaster, Hunters can be useful due to their ability to critically hit at range. They can do so for free at 70' with the Nightspear or Deadlyrang, or 80' with the Sword of Zar from BT2, and can get 90' criticals (with limited charges) with Aram's Knife, Zen Arrows, or Ag's Arrows. Also, give a Hunter some Black Arrows (and a bow), and you can now critically hit entire groups at once (and Black Arrows are more common than Deathhorns in my experience). The one catch is that you may need to gain extra levels, as enemies in BT3 have much better saving throws than those in BT1 and BT2.

By the way, Hunters are also critical in classc 16-bit versions (DOS and Amiga); some endgame enemies (Rock Demons) are basically immune to spells, and Hunters are the only ones who can critical hit without spending a turn hiding. (Weapons with petrify and death effects are broken (in other words, they don't work) in 16-bit versions, and using a summoned or spellbound monster won't work as status ailments from monster attacks (on both sides) are *also* broken, as are monster breath attacks.)