So question:

Is it possible for me to roll multiple 18's in MM1 character creation?

Just trying to get the best group possible together.

Does MM1 use best 3 of 4 dice? If it uses 3 straight dice, then chance of getting 18 is 1/216. If it used best 3 of 4, which I believe it does, chances of getting 18 are 1/72.

If it's 1/72, then chances of getting 18 in 2 stats is 1/(72*72), roughly 1 in 5000. If you make one roll in 2 seconds, that's more than 2 and a half hours of rolling.
Chances of getting 18 in 4 stats is 1/(72*72*72*72), about 1 in 26 million.

Just aim for decent stats. It won't make a big difference in the long run.

I would add on top of that, in MM games (including MM 1), 17 in stat gives exactly same benefits as 18 in stat. IIRC there are very limited permanent power-ups in MM 1 and they are +2. 19 provides same benefits as 20. Thus aiming for multiple 17 in important stats might be more effective way of creating characters.

As others have said, having really good stats at the beginning is not that important. Just make sure you have decently high numbers (like 15 or higher, maybe?) in the relevant stats for each character. Speed is surprisingly helpful for all characters, since it lets them act first in combat, so try to make sure your characters have at least a reasonable score in that in addition to their primary stats.

Permanent stat boosts are not as common in MM1 than they are in MM2, but, without going into spoilers, there's more opportunity for permanent stat boosts than Sarisio suggests. Later on in MM1 your characters will all have boosted stats.