The bride definately wants 3 real tiers of cake. What size pans would you use to come close to the servings and still look nice and proportioned?
You look her straight in the face and tell her "You can't have a 3 tier cake for only 50 people, ESPECIALLY if you're using square cakes."
A 10" square serves 50 by itself.
An 8" square serves 32; a 6" square serves 18 = 50 servings.
To me, a 4/6/8" cake would look like a "toy" cake. If she is specifically wanting a 3-tier cake, then it gives me the impression she is wanting the look of a "real" cake (i.e. real as in edible.... real as in size) and she would be disappointed in the tiny 4/6/8 cake.
Unless you can show her dummy cakes in the 4/6/8 size and get her to sign off on that size, then she needs to explore other options.
I just have a low tolerance level for brides who want a grandiose 7-tier cake "....but only for 25 people!" They want the BIG cake with the TINY price ... that's all they're saying.