The correct order of relative basicity of amines in the gas phase is 3º > 2° > 1º > NH₃. The alkyl group releases electron and thus, tends to disperse the positive charge of the alkyl ammonium ion and therefore stabilizes it. Since, ${\mathrm{NH}}_4^{+}$ (from NH₃) has no such alkyl group, it is not stabilized to such an extent as alkylammonium ion.