I have this situation can I combine the following terms
DDDa = |d1-a1-p1/C|+|d1-a1-(p1+p2)/C|+|d1-a1-(p1+p2+p3)/C|
if you are 100% certain that none of the terms will ever be negative, then the | |'s aren't needed and you can combine them. I suspect that's not the case.
Likewise if you are certain they are always negative you can also combine them but again that's unlikely.
So the answer is probably no you can't.