I have 36 stitches in the round
Then it says commence inc 2 stitches underarm
every 8 round 3 times =42
Where would underarm be is it the seam beginning of round
What is the name of your pattern and designer?
Are you knitting the body of the sweater or a sleeve?
If there are no designs or colorwork to center then either side of the beginning of round is a good place to set the increases. Sometimes working the increase one stitch in from the marker on each side is best.