New roof installed, but damaged wood not replaced?
In the contract it specified removal of all shingles and repair of any damaged/rotten wood.
From the looks of it, that sheathing was likely damaged by the new shingle nails. The roofer probably didn't know it occurred. At least two of the boards are broken at knots and I can't tell about the other one.
Since you have attic access, take a piece of 1x or plywood and screw it to the bottom perpendicularly under the broken boards after pushing the pieces back in position. Make sure it's long enough to span the broken board and the ones to either side. Use screws short enough that they don't penetrate to the shingles.
The roofer should be willing to come back and take care of it. I'd call, but if they won't come, it's a pretty minor fix.