It looks like you're trying to express something mathematically, but the notation is incomplete. In the expression "S.A. = __ + (S1 + S2 + S3) __", the blanks need to be filled in with appropriate mathematical symbols or terms.
If "S.A." stands for "Sum of Areas," for example, you might fill in the blanks with terms related to areas or coefficients. If you provide more context about what S.A. represents and what you want to calculate or express, I'd be happy to help you complete this equation!