How much batter should I use to bake cake using 3" round pan. I can't seem to get it right. I've been told half-full and also that I can use an entire box mix. It either comes out to short to tort or doesn't cook thorougly. Help!

Batter amount depends on the size of the pan, as well as the depth. If you want consistant results, it's best to actually measure how much you're putting in the pan. Here's the Wilton chart for how much batter to use in various 3" pans:

http://www.wilton.com/cakes/making-cakes/baking-party-cake-3-inch-pans.cfm

HTH

Are you talking about a 3" deep or a 3" diameter pan? One whole cake mix is 5-6 cups of batter and fits an 8" pan that is 3" deep. The box says two 8" or 9" pans which are usually 2" deep. A 6" pan will take 2 cups if it's 2" and 3 cups if it's 3" deep according to Wilton. Basically fill it to where you think it's right. If you find that half full doesn't give you the height you want, then fill it 2/3 or even 3/4 full. If you're finding it's not baking in the allotted time, then you could try putting a flower nail in the pan to help distribute the heat. Also, depending on what you're baking at, if you're finding that baking it longer is making the cake tough on the outside, you could try dropping the temperature by 25 degrees. It will obviously take longer to bake, but it may yield the results you are looking for.