A bride placed an order for 120 servings and we designed a 3 tier round 12", 10", 8" cake. Now that it's 3 weeks before the wedding she says she only needs 90 servings. How can I transfer the design to only serve 90? Scaling down to 10", 8", 6" won't yield enough servings. Will it look super weird to do like 12", 8", 6"? I've done uneven tiers like that before but only with 4 tiers where the difference between the bottom two tiers and top two tiers were at least the same. HELP!!

..............Will it look super weird to do like 12", 8", 6"? .........

Totally NOT I've done dozens of that size over the yrs. It does help a bit if you have 9" pan to make that instead of the 8, plus it give you 8 more servings
Are they serving the top tier? Or saving it? Otherwise a 12,9 only give you 88 servings.
Let the bride know the best you can do on servings is either 88 (w/o the top served); 100 w/the 6"er or stay w/the original size and have cake leftover. It's her call which way to go.
Inform her leftover cake can be cut into 4 or 6 or 8 chunks and sent home w/family members; friends etc. It can be fzn for some near-future special occasion or just a great dessert next week

Do you have 9" pans? 6, 9 and 12 is nicely balanced-it was always one of my favorites to make and decorate.

I agree with 12-9-6

Just out of interest, is she still paying for the contracted amount?