The oxidation numbers of nitrogen in nh4+, no2, and nano3 are, respectively: -3, +4, +5 +3, +5, +4 +3, +5, -4 -3, -4, +5

Hydrogen has a +1charge and the whole thing has a +1charge so
(N)+4(H)=+1
and plug in H you get
(N)+4(1)=1 which means N has a charge of -3
If you use that set up for the rest you should be able to figure them out if there is no overall charge you just set it equal to 0