You are not missing anything. These cameras should be possible to match in 1080/23.98. As long as you don't need 4:4:4, 24.00fps, 23.98/24.00 system setting, P10log output, variable frame rates, etc. that are 3700 only features (as you mention)
I've used both quite a bit, and the 3100 has some sort of noise reduction built in (adjustable) + a newer CCD so you might need to work the matrix a bit to make them look the same.
Might also want to dial 'down' the new improved audio bit rates of the 3100 to match the 3700 audio format.
Aasulv Wolf Austad, fnf
Director of Photography
Los Angeles based