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.

Thanks