It really depends on your level of comfort and the comfort of the resident rat.
He may feel overwhelmed by the two newcomers, he may be less inclined to be aggressive if there are two.
Just do whichever you choose and be sure to go slow and go back a step or two if any fights break out.
I have a male nearly 2 yeaars old(it's a guess, he's fully grown and has no intrest in females so I'm assuming he's older) and I was able to fly through the intorductions with no problems, but others have had maor setbacks.
I had problems mostly with my girls. Right after I got two new boys, my brother accidently put them in the wrong cage. They were in the cage with my resident male, Dobbie, whom I was going to start intros with later. Dobbie didn't hurt them at all, he just groomed them.
But it could have turned out a lot worse. Had it been the other male that was here, he'd likely have injured or killed one. It was just his personality. He doesn't much care for other rats or people.
Depending on where you are getting the rats from, it might be a good idea to get one at a time and introduce them to your male. It depends on the travel involved. Or if your cage is big enough you could get three new ones and introduce one to your boy, during which the other boys will have each others company, and then later introduce one or both of them.
I would base that decision on how well they interact in neutral territory. If they seem to get along well, then sure, why not. If they are ganging up on your resident rat or there is too much fighting, perhaps do it one at a time.