Set 3(theta + pi/4) = certain special angles, such as 0, pi/6, pi/4, pi/3, pi/2, -pi/6, -pi/4, etc to find theta.

Example :

3(theta + pi/4) = 0

theta = -pi/4

Then subs the value of theta back to the equation to obtain y.

Plot the graph

EDIT :

it will be much easier if you use transformation of graph

1.start by drawing y = sin theta ---------------------------------> you get y = sin theta

2.multiply all the values of theta by 1/3 --------------------------> you get y = sin (3 theta)

3.translate the graph by 3pi/4 unit in the negative x-direction -----> you get y = sin (3 theta + 3pi/4)

4.multiply all the value of y by 2 ---------------------------------> you get y = 2 sin (3 theta + 3pi/4)