Which has greater -I effect -N(CH3)3 or -NH3 ?
If you think about the +I effect of ch3 then the answer would be -N(CH3)3 has less -I effect than -NH3 .
If you mean ED (Electron Donating) by +I and EW (Electron Withdrawing) by -I your answer is true!!!
If you want an explanation for this fact here he is:
Hydrogen is less ED or +I than the Methyl Group. Therefore replacing he three Hydrogens with a Methyl Group (Forming triMethyl Amine) enchances the Basicity of the Nitrogen Atom becoming more ED thus more +I who also means less -I effect!!!