The amine X can be?
!!CH_3!! is an electron releasing group or we can say it has !!+I!! effect. So the electron density over !!N!! atom increases and its tendency to donate increases.
So as the number of methyl groupds attached to the !!N!! increases the tendency of !!N!! to release electron increases.
But in the case where !!3!!, methyl groups are attached to !!N!! the steric hinderance is significant and reducing the chance of !!(CH_3)_3N!! to react with diborane.