Excuse my in-experience, but how much does it cost to produce a paste stencil?
And would it be feasible to have stencils made for pwbs that tend to grow or
shrink, as the case may be?

This would help, especially if board sizes vary from batch to batch, or vender
to vender.

Granted 3 separate stencils sounds like a waste, but it would get around the
problem.