I covered 3 cakes in fondant but not stacked. 12'' 8'' 6'' the 12'' is leaning but I cut pieces of cardboard and stuffed it underneath the leaning area, my question is would it be ok once I stack the other tiers..

I'm confused as to hom a single tier of cake could lean. Do you mean layers are sliding off? It's just uneven? Sorry, I genuinely do not understand.

uneven...it baked uneven and I tried to cut evenly, but it didnt work. so i figured i can probably fix it by adding more frosting to uneven side and also by adding cardboard underneath to even it out..

Ok, so it's not leaning then, it's just uneven? It's not going to fall over? Sounds like you just need to ice it evenly. Don't ice to the shape of the cake, ice to the shape of your board. Hope that makes sense. Look up Sugarshack icingmethod sometime.

If your support system is correct, then it won't matter that the cake is leaning except for aesthetically. And of course you should do as AZ said and ice the correct shape even if your cake isn't. I would not use cardboard shims at all.

thanks!!!