The correct order in aqueous medium of basic strength in case of methyl substituted amines is:

In aqueous medium basic strength is dependent on electron density on nitrogen as well as solvation of cation formed after accepting ${\mathrm{H}}^{+}$. In protic solvent like water, the order of basic strength is

${\mathrm{Me}}_2\mathrm{NH}>{\mathrm{MeNH}}_2>{\mathrm{Me}}_3\mathrm{N}>{\mathrm{NH}}_3$.

This happens beacuse in case of ${\mathrm{Me}}_2\mathrm{NH}$ there will be optimum (+I) and solvation while in ${\mathrm{MeNH}}_2$ has more significant hydration than ${\mathrm{Me}}_3\mathrm{N}$.