Here it says what are the possible values for NNL for an electron found in the third principal level and D sub level. Alright, so here they're saying third principle level. That means third shell. If we're talking about principle, we're talking about our end quantum number. So this would be N=3.
And then remember if we know the sub shell letter or sub level letter we know what L is. Remember here 0,1,2, </