Consider Group 5A elements: nitrogen, phosphorus, and arsenic. These elements show an increase in their atomic numbers. Which element has the highest ionization energy?
Clearly, nitrogen will have the greatest ionization energy.
The ionization energy is the energy associated with the formation of 1 mol of gaseous cations, and 1 mol of electrons, from 1 mole of gaseous atoms under standard conditions.
The ionization energy is well-known to increase across a Period (from left to right as we face the Table) because of the increased nuclear charge, BUT decrease down a Group, down a column of the Periodic Table, because the valence electron is farther removed from the nuclear core.
For your sequence, nitrogen, phosphorus, and arsenic, arsenic, the lowest Group member, will have the least ionization energy, and nitrogen will have the largest.