Moderating variable

[Pvunderink]

Dear forum-members,

I've got a question about my moderator variable (firm size).
So far I've introduced a 2-way relation between CSP and Firm size to FP (dependent variable). Like: FP=b0 + b1CSP + b2Firm Size + b3(CSP*Firm Size)
However, I'm not sure how to make the regression formula with multiple independent variables. Can someone explain this to me.
In this case I've got: FP= b0 + b1CSP + b2KLD +b3Firm size + ?????
How many interaction variables do I have. Is it only b4(CSP*Firm Size) + b5 (KLD*Firm Size) or also b6(CSP*KLD) and b6(CSP*KLD*Firm Size)?

Regards,

Pvunderink