Ok, ive completed everything else, all the sidequests, and my core party is celes edgar sabin terra, they are at lvl 70, How should i split my guys for the 3 parties, and what levels should the others be, as well as what spells are best to learn for them?

Level 70 is actually extremely high. As long as your other guys are around 40, you should be fine.

As far as splitting the parties goes, you can pretty much mix and match however you want. I know that I tried to have at least one powerful magic in each party, as well as one pretty heavy physical attacker. Just don't put all of your most powerful characters in one party.

Yeah, i split them up, and i raised their stats so that they are both strong in magic and physical.

You'll be alright, then. I do remember that I left Gau and Relm out, though, and took Umaro and Gogo instead.

You may end up leaving the Tower one or more times though, if you're still working on the Dragons or are planning to get any more of Gau's Rages. If you want a rare Blue Magic for Srago, make sure to have him in the party when you're fighting the Goddess.

If you don't care about any of this stuff, then just go with whatever you like.