This looks like a homework question. Our homework help policy can be found here. We mainly just want to see what you have tried so far and that you have put some effort into the problem. I would also suggest checking out this thread for some guidelines on smart posting behavior that can help you get answers that are better much more quickly.
Hmm ok well I think it has something to do with how the coefficients are calculated when there are more variables added. I looked at the coefficient calculation for a single variable model and a two variable model. When you calculate the coefficient of one variable it also depends on the other variable. I'm not sure how it looks like for 3 variables, I've tried searching for it but I can't find anything. Can anyone shed some light on this for me? If anyone has a general equation readily available for me to look at I think that would help me answer my question.