More the electron density on $\mathrm{N}$, higher will be the basicity. Density on $\mathrm{N}$ is influenced by the (i) nature of the group (+I or -I) present in alkyl group or benzene nucleus and (ii) resonance (delocalisation of the electron present on $\mathrm{N}$ ). In $\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{3} \mathrm{~N}$ :
electron pair is delocalised to the maximum extent due to three benzene rings and hence least available for protonation, thus it will be least basic)