Let
A = location of buoy 1
B = location of buoy 2
C = location of buoy 3
True north is 0 degrees on a compass bearing. Turning to 135 degrees means you turn to the south east direction exactly. Then we can break the angle down as shown in the diagram below. We have the angles 90+45+45 = 180 along the right side of location A.
Due to the fact we have parallel lines, specifically the north/south parallel lines going through A and B, we know that the green angles in the diagram are congruent. Both of which are 45 degrees. The purple angle of 45 degrees is to indicate turning to the bearing 045 degrees when you get to location B.
The green and purple angles add to 45+45 = 90, so angle ABC is 90 degrees. Triangle ABC is a right triangle.
We can use the tangent ratio to compute the angle ACB which will help us find the bearing from location 3 to location 1.
tan(angle) = opposite/adjacent
tan(C) = AB/BC
tan(C) = 4.2/2.8
C = arctan(4.2/2.8)
C = 56.3099324740202
Make sure your calculator is in degree mode
This is angle ACB. It combines with the purple angle of 45 degrees that is adjacent to point C. So we have 45+56.3099324740202 = 101.30993247402
Then this adds to 180 as this is the measure of the red arc near point C.
So, 101.30993247402+180 = 281.30993247402
The bearing is approximately 281.30993247402 degrees
Round this however you need to.