Or try this more recent patch:

http://www.ttlg.com/forums/showthread.php?t=138607

...which includes the patch linked to above, plus quite a bit more.

I found the level design in Thief 3 to be awful. I got completely lost in that first castle.

Yea, I felt exactly the same way at first. The problem is made worse by an utterly useless map. But I persisted (playing on easy and simply killing everybody) and found it to be quite enjoyable because it forced me to check every nook and cranny and be very thorough.

Having load screens right in the middle of the level blows donkey balls though and is a immersion breaker to an otherwise very atmospheric game. Stick with it, once you learn the castle layout (not as big as it looks) it's a lotta fun.

I did that, even flicked that switch. What was is that had to be done in order to progress?

After flicking the the torch switch in the bedroom you go to the basement (and thru yet another load screen, back to the area where you first start the level) and clean out the safe. Then it's off to the main gate outside near the lion head and the level ends.

I installed T3 with win 7 OS, it refuses to launch. Advice?

yes. ignore the false positive from your AV and use the latest patch anyway.

patchers can trigger such warnings easily, as they modify files (that's their purpose you know), and overzealous AV software can jump the gun and mark them as a threat (using a primitive logic "it modifies files so it must be a virus"). if the patch is posted (by someone who has been a member of the community for a couple of years) on a prominent forum and a few dozens of people have confirmed that it works nicely (myself included), it should be pretty safe to install.

your call.

I loved the first two, I even finished playing through Shadows of the Metal Age. I honestly find it hard to understand how I could not get into the Thief series when I was younger.
I've read some scattered bits here and there about obnoxious load screens a la Invisible War and a goofy rendition of the City.

Tell me straight, will I enjoy DS?

Also, aside from what voodoo47 posted (http://www.ttlg.com/forums/showthread.php?t=138607), are there any other fan patches that I should/need to download?

Thanks for the help!

I'm taking the blank link posting as a "Go for it!".

Actually, nobody can tell you that. It still polarizes even die hard fans.

Personally, I recommend to play it just for the experience (plot, atmosphere) and the stuff that is considered better than the older two games (physics engine, dynamic shadows and graphics).
The Cradle level is a must-play like many have expressed already and other levels are also very good. I surely wouldn't want to miss out of those.
On the other hand the levels are way smaller than in the first games and it is definitely dumbed down a bit for consoles (Xbox1 and PS2 capabilities) but one can live with than and there are some interface tweaks for PCs available. The only real problem I experienced is the "jump bug" where Garrett kinda slides around after doing certain jumps. They never fixed it, sadly.
And, yes, there are also some fan missions for it - playable with the GarrettLoader tool. Mind you, because T:DS received mixed attention from fans it never had the masses of missions like T2 so your time with it will always be limited ;-) TTLG is usually at the cutting edge regarding mods so I can't really add to this.

However, if you are through with T:DS then check out "The Dark Mod" for Doom 3 - while it has no real Thief storyline you may come to the conclusion that it is a better platform for "thiefy missions" than T:DS...

coming from skyrim or so this really is challenging at first, but you got the clue after 15 minutes.

Agh!

What Garrett face/Garrett outfit? I think I'll start with vanilla and not do the stuff from this texture pack at first, but at some point I would want it to be upgraded.