He started working as a bank clerk 3 months ago.

S + began/ started + V-ing / To V …  in + time (May, 1990,…)

                                                             time (years, month,..) + ago       

S +   $\left\{\begin{array}{l}have/has + Ved/3\dots \dots \dots \dots \dots ..\\ have/has + been + V-ing\end{array}\right.$$\left.\begin{array}{r}\\ \end{array}\right\}$ since/ for + time