Wedding Cakes Sizes

Would a 3 tier wedding cake(sizes 12,10,8in)feed 50 guests?

I want to cut the pieces kinda big.

That should be plenty! My wedding cake is 12-9-6 inch tiers to feed 80 people with average slices (we're having it after dessert as part of a late night sweets table). So I think yours should be plenty for 50 people with bigger slices. Best Wishes!