Volume 3 needs to start with #16 as Volume 2 left off with #15.  I'm not sure why MTIO #6-7 would be included... they feature Dr. Strange and then Valkyrie, but are they direct crossovers with the Defenders issues?  If you go that route, you'll have to include all of the individual members' appearances in Marvel Team Up and Two-In-One down the road.

I was actually going to take a shot at reworking this line soon once I finish posting my updated mappings for the What If Bronze Age Page 1 titles (Silver Age lines that continue into the Bronze Age).