I have a cake to make for my cousin's baby shower. It needs to be 3 tiers. Would 10, 8 & 6 look ok? It needs to be square. The number of servings do not matter because there will be extra with a three tiered cake!
How many people are you serving? You think is doesn't matter, but depending on your needs you could make some of the tiers out of dummies.
I always prefer 3" difference for "ledge" decorations otherwise it tends to look crowded so a 6" @12 servings, 9" @ 27 servings and 12" @ 60 servings. But "Whoa Nellie" that is a ton of cake and I have never seen 99 people at a baby shower
Perhaps making the bottom tier a dummy you will have 39 servings. Top tier and bottom tier dummies would give you 27 servings about the right amount.
And added benefit, you can reuse the dummies!