Dear dark,

Have you already considered?
Case 1: P1 on, P2 on, P3 off (spare)
Case 2: P1 on, P2 off (spare), P3 on
Case 3: P1 off (spare), P2 on, P3 on

As per your explanation, I guess you only did let say case 1. If you run all 3 case above, is it always fail for spare pump?

In my opinion Exp, joint is the last choice for the case. I ever did with Exp. joint. But it was connected to existing line, no space too crowded.