Answer
The result of protonation and its resonance forms are shown in the picture below.
Work Step by Step
When Nitrogen #1 is protonated, it becomes positively charged. Since there is no double or triple bond on an atom next to it and no non bonded pair of electrons, in order for resonance to exist.
When Nitrogen #2 is protonated, again same as nitrogen #1, there is no double or triple bond on an atom next to it and no non bonded pair of electrons, in order for resonance to exist.
However, when nitrogen #3 is protonated, there is a double bond next to the positive nitrogen, making the delocalization of positive charge possible and forming the resonance structures.
