Ok so I want to make a three-way character with 3 different classes or kits or whatever, please don't ask my why or warn me or something.
So is there a mod to make custom classes? without the premade ones? just adding classes i want into this one kit
or just a software or a 'legitimate way' to make this

Thank you, all responses are greatly appreciated !

Not in BG1, unless you install either TuTu or BGT to run it in the BG2 engine. Even for custom classes, the BG2 engine restricts you to one kit, which goes to your primary class. Usually, only Elves or Half Elves may take 3 way multi-class. You would have to do some research on any mods available the change things to the way you want. Look on the big mod sites.

I've used ShadowKeeper to add a Kensai kit to my F/M/T, and he was every bit as awesome as you might think.

There is a way to make multiple kit characters, which I found in my digging a while back. I believe it was the Spellhold Studios modding site. It seems to be a good, valid mod, and some screenshots showed the proof. Looks like a fun idea, and one I may try eventually.

EDIT: I just reread your post, and it seems you may want "aftermarket" classes, not just the mix and match of existing classes/kits as I posted above. I have seen some sites that have fan made classes, like a "dark paladin" or something like that. Also I've seen variations of monk as well. A search engine would probably find you what you need. Sorry I wasn't of more help.