The alkyl groups are electron releasing group $$\left( { + {\rm I}} \right),$$ thus increases the electron density around the nitrogen thereby increasing the availability of the lone pair of electrons to proton or lewis acid and making the amine more basic. Hence more the no. of alkyl group more basic is the amine. Therefore the correct order is $$N{H_3} < C{H_3}N{H_2} < {\left( {C{H_3}} \right)_2}N$$