Someone wants me to make a movie theatre theme cake for 120 people. What size should I make it for 3 tiers? Or 2 tiers?

You didn't say what shape cakes, so I'm going to assume round. Here's round cake serving guide for you to save:

4 -- 6
5 -- 10
6 -- 12
7 -- 16
8-- 24
9-- 30
10-- 38
11-- 45
12-- 55
13-- 65
14-- 75
15-- 85
16-- 100
18-- 125
20-- 150

***1"x2"x4" servings***

Thanks prterrell this really helps me too!! I never thought they could get enough slices out of each cake, so I tend to over do...

Remember: the proper way to cut a cake is not wedges, but squares.

another chart to keep that includes other shapes

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

I usually go for 4 tiers. 12, 10, 8, 6.
Edna

For three tiers I would do 15,11 & 7. For two tiers I would do a 16 and 10. These would be 2-layer tiers. You didn't say if you were doing 2-layer tiers or single layer tiers. That would make a difference in servings, also.