The increasing order of enthalpy of vaporization of NH3, PH3, and AsH3 is
PH3, AsH3, NH3
The inter-particle forces are strongest in NH3 (Hydrogen bonds). In PH3 and AsH3 the inter-particle forces are dipole-dipole forces which are relatively stronger in AsH3. So the increasing order of vaporization is PH3, AsH3, and NH3 .