What name have you decided on for your new baby?
If you're getting an opportunity to visit your new furkid in advance of bringing him home, Id suggest taking with you a few pieces of cloth, so you can rub them over your new kitten, then take them home, so your boys get used to the new scent in advance. Similarly, rub them over with a cloth and leave it with newboy. Thatw ay they get a chance to familiarise the scent in their world, and may find it less disconcerting when Newboy arrives home.
Generally with 3 cats, you stand a good chance of choosing a personality that will match with one or other of your cats, and as you seem to have "one of each" at the moment, Newboy should gravitate to one of them if not both of them.
Moray was horrified when he first met the girls - they were hiss-spits and ill mannered ruffians from the wrong side of the tracks. He spat and hissed a fair old bit himself, but he was also intrigued by them, as he's basically a very social cat (when he's not busy sizing up his neighbourhood). It didn't take long at all for them to take to Moray, and him to them. The trouble has been in getting them to take to human contact, though they're much much better than they were. At least you shouldn't have that difficulty with yours.