https://www.teamrankings.com/college-football/stat/third-down-conversion-pct
These stats show that NU is 61st at 39.5%.
NU is getting chunk plays on 1st and 2nd downs to produce scores. Against CU, NU had 2 drives where it scored a TD without having a 3rd down in the drive, and another TD drive with only 1 third down. Against Northern Illinois, NU had 3 TD scoring drives without a 3rd down.
Looking at the drive charts from the first 3 games:
South Alabama - 4 three and outs in 12 drives
Colorado - 5 three and outs in 15 drives (all in 2nd half and OT)
Northern Illinois - 2 three and outs in 14 drives
So, yes the three and outs were concerning in the first 2 games, but NU was much better against Northern Illinois.