I think you guys would be best to target a 3C in free agency than to lose what assets you have trying to trade for one. A guy like Haula out of Florida might be a good target for you. He's fast, talented and works hard altough he does have a tendency to get injured so he won't cost too much, maybe 3 million dollars a season on a short deal. Don't know if he will be available though.
Cody Eakin could be had too, although I'm not too sure Winniped will let him go with their weakness at C. Derrick Brassard also played relatively well as a 3C with the Islanders and would probably be quite cheap to sign on a two-year deal.
Johan Larsson would be a solid grab too as he is really underrated and wouldn't cost more than maybe 2 millions. He would be fabulous as a shutdown 3C for you guys, the way he plays really reminds me of Marcus Kruger. But he was in the doghouse in Buffalo for no reason at times during the season so they likely won't re-sign him even though he was their best defensive forward last year in my opinion. Their potential loss, your gain.
And you've also got a couple options out there for trade, but they've already been mentioned.
Anyways, don't get too down and lose hope, your team is on the upswing. Just shore up that third line, make some small adjustments on D, draft a good goalie, and you'll be good with progression from your young players.