Adults know they shouldn't attack puppies, but unless he sees you stepping in to stop any ganging up/mugging going on, he's going to need to defend himself. So best I can advise (because I'd not have taken on two puppies at the same time, without lots of experience first) is you have a place for your older dog where he can take a nap without feeling threatened, and have a couple of crates for these puppies so when you are not there to control the situation, they can be safe. Small wonder this poor Lab. is reacting - and unless you step in and take charge, he will attack.
It's not going to happen overnight and of course, they must be together to bond, but only under controlled circumstances. Hopefully once he sees you are in charge, he'll relax and settle down.