Basicity of amines increases with increase in the number of R groups, then why is  N,N Dimethylmethanamine considered less basic than methanamine and N-methylmethanamine?

N, N-Dimethylmethanamine is less basic due to two reasons :

1) It is a tertiary amine with no H on the nitrogen atom which leads to less stable ammonium salt of this amine.
2)It is sterically more hindered and is less solvated in water and hence less stable ion

Both the factors mentioned above make it less basic as compared to primary and secondary amines.