If you do the BOs of 8 sts at the armhole edge (3x) you're going to end up with 'steps' which you may not like or find difficult to join when seaming. Realizing this is your first sweater, you might consider doing slipped transitions so you'll get a sloped edge OR short rows (perhaps you've done those on socks?).
How are you planning to join the shoulders? If you have live sts you can do a 3-ndl BO which is a strong, really clean shoulder seam. If not live, you'll have to seam.
If you're planning to seam, and want a sloped edge, work to w/in one st of the BO edge (for right armhole that's the WS) and slip the last st. Turn. Slip the first 2 sts (the slipped one and the next one) onto the right ndl and work one BO. Work the remaining seven BOs.
is image of the steps that you'll have if you do standard BOs.