Hi!
This weekend I'm making a 3 tier for my daughters 18th bday. I need a really really good firm yellow cake mix recipe (scratch or otherwise), to stand up to a 3 tier. I looked in the recipes here but I was so overwhelmed and i'm new to cake making so they all look alike.....
if any of you can suggest a really great recipe you have encountered, I would appreciate it SO SO much! thanks..... 
Welcome to CC!! 
I highly recommend the WASC (White almond sour cream) cake that is posted in the recipe section here at CC. http://www.cakecentral.com/cake_recipe-2322-0-White-Almond-Sour-Cream-Cake.html
Most of the time I use whole eggs in mine which makes it a slightly yellow cake. This is also a very large recipe, makes about 14 cups of batter. You are going to need a lot of batter for the cake you are making your daughter..........sounds like lots of fun!! 
Happy baking and glad you found CC!
I also love the WASC but for classic yellow, this is a MUST try!:
http://www.cakecentral.com/cake_recipe-1972-Durable-Cake-for-3D-and-Wedding-Cakes.html
It is my most common cake when people are thrown off by the almond and sour cream. Rave reviews every time and soooo easy!
I'm also a fan of the WASC cake in all its flavor variations.
Expanded flavors version of WASC cake recipe:
http://tinyurl.com/2cu8s4
(Using DH white cake mixes, the full recipe yields a tad over 14 cups of batter. Which as Teekakes pointed out is great for large or multiple pans.)
Original WASC cake recipe:
http://forum.cakecentral.com/cake-decorating-ftopict-584319-.html
Here's another recipe similar to the one kymscakes suggested (but uses buttermilk):
http://www.cakecentral.com/cake_recipe-6948--Enhanced-Mix-Recipe.html
Also, the weight of the higher tiers aren't supported by the lower cake layers but by any number of different cake support systems.
Illustrated common cake support systems:
(With complete and accurate directions.)
http://tinyurl.com/y22z72
More on SPS system:
http://forum.cakecentral.com/cake-decorating-ftopict-561846-.html
(See leahs post.)
Illustrated level dowel cutting (by indydebi):
http://forum.cakecentral.com/cake-decorating-ftopict-434013-.html
How much filling to use (chart):
http://tinyurl.com/2gay6n
HTH
those recipes are awesome..... the party is saturday...
ok, here's my plan. i'm thinking of baking the cakes tomorrow. putting the filling and buttercream on thursday, then the fondant on friday and letting it set and sit til saturday....what do you all think? am i letting too much time elapse? and also....do you guys have a good filling recipe?
you're being so helpful...thank you.
soly
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