Question

Three Blocks attached by massless string... find the acceleration. Do we have to do this for every type of problem or would I also get the same answer if I kept it as (T(subA)-m(subA)g=m(subA)a?

Can someone please explain the steps or clearer way to solve for acceleration? I know they chose the direction of acceleration to be positive for mA but this confuses me because I'm not used to doing it this way. Do we have to do this for every type of problem or would I also get the same answer if I kept it as (T(subA)-m(subA)g=m(subA)a? If possible can you please tell me if I am on the right track (see pic below)? Thank you!

enter image description here

enter image description here