Originally Posted by
alias5000
Is it fair to assume that the chainring tooth spacing between the three chainrings should be equal? (Likely yes!?)
Then I can just measure the difference and shim accordingly
It's a decent starting place, but may not be that exactly. Your OP is a bit ambiguous in the details.
On one hand you say the chain overshifts going from large to middle. That implies that the teeth are a bit outboard (closer to large ring). OTOH, you say it hangs up in between. This is common when the gap is too large allowing the chain to ride ride the tops of the teeth while leaning on the face of the outer ring. If that's the case, then the teeth are too far inboard, (the opposite of the first situation).
So, the first step is to decide whether the chain is over or under shifting, vis a vis the middle ring. If it's consistent one way or the other, you know what's needed. However, if it's not consistent, you need to consider other possibilities.