how to mark polarities to write loop equations
According to Passive sign convention, polarities are according to following scheme:



Assuming directions of currents as in following images, we'll write following polarities while writing equations for different loops.

Loop 1:


Equation:
$$
    I_{1}R_{2} + (I_{1}-I_{2})R_{5} +(I_{1}-I_{3})(\frac{1}{sC_{1}} + R_{1}) = 0
$$

Loop 2:

Equation:
$$
I_{2}R_{4} + I_{2}\times{sL_{4}} + (I_{2}-I_{3})R_{3} +(I_{2}-I_{1})R_{5}=0
$$
Loop 3:

Equation:
$$
(I_{3}-I_{1})(\frac{1}{sC_{1}} + R_{1}) +(I_{3}-I_{2})R_{3}=E\times{sin(\omega t)}
$$