The dipole moment of the system shown in figure is 

+3Q can be written as (+2Q+Q)

+2Q and -2Q form a dipole whose moment is $p_1=2Q \times 2l=4Ql (\leftarrow )$

+Q and -Q form another dipole whose moment is $p_2=Q \times l=Ql (\leftarrow )$

$$\begin{gathered} \therefore \overrightarrow{p}=\overrightarrow{p_1}+\overrightarrow{p_2} \\ \Rightarrow p=4Ql+Ql=5Ql \end{gathered}$$