i've been asked to do a 3 tier square cake. it needs to fit 100 people. what size pans do i need? please can some one help!

I would do a 14" - 10" - 6". Not counting the top tier since it is usually saved for the anniversary, that should give you 100 servings. Depending upon who you ask & what serving charts they use, they might say it serves a little more, but I like to error on the side of caution.

......AZCakeGirl's answer is for American cakes but since you are in the UK are you using Fruitcake? I understand those serving sizes are much smaller so you would get many, many more servings from the sizes mentioned.

I use the Wilton chart for tiered cake servings so for the 100 servings you need a 12, 8 combination would give you 104. If you want to add the 6" top tier that is not counted in the servings, that's fine This # is based on a serving being 1"x2"x4".

Oops, sorry! I didn't notice you were in the UK & had different types of servings on your cakes there.

for fruit cake you'll need a 6" on a 10", check out this site or any other UK wedding cake site.
http://www.maisiefantaisie.co.uk/serving-information.html

thanks everyone for your help. i actually use a sponge cake as people i deal with don't like fruit cake. but i found the chart requierd. thanks for your help everyone!