The work done (W) when the gas expands at constant pressure P(=3 atm) from volume V1 to V2 is given by
W=−∫V2V1P dv
=−P∫V2V1dv
=−P(V2−V1)=−PΔV
where, negative sign indicates work done by the system.
Here △V=3L
We know, 1 L atmosphere =101.325 J
Thus, W=−9×101.325 J=−911.925 Joules