The numbers correlate to the personnel groupings. The first number is d-line, the second is LBers, the third is DBs (defensive backs, i.e. CBs+SS+FS).
So in a 3-4-4 for example you would have 3 defensive lineman with their hands in the dirt, 4 LBers, and 4 DBs (2 CBs, 2 safeties). Of course, in a 3-4 alignment one of your LBs will typically
always be
acting as a de facto d-lineman by blitzing.
Generally speaking, in the NFL you would alter this to match how many WRs the offense is showing. If they come out in a 4 WR set, you'll want 4 CBs to cover them, so you'd match them with a 4-1-6 (dime).
In MFN, many people would typically favor nickel or dime sets even against sets with fewer WRs because LB play was so bad. With the new engine however this should hopefully see a dramatic improvement.