Matthews and McDavid might be underpaid really considering the impact they bring. Nylander is market value, maybe even cheap too considering on-ice impacts. Would it have been nice to get them both well under market value at 500k-1m cheaper? Sure, but that's not cap killing. Cap killing would be signing league min quality players to mid-tier contracts. Almost every team has those, except the leafs really.
I do think there was a bit of a misstep to not get marner around 1.5-2 milly cheaper, though from the sounds of it he was never signing for less. So it is what it is. You can't change what players want or feel they're worth. At the end of the day these contracts won't kill a team. If you feel the contracts are the reason the team lacks success you need to look elsewhere.
In a very best case, dream scenario they save maybe 2-3m per year on the three deals. Really hard to say they can't be competitive because of 2-3m less cap space. If they continue to lack success and JT starts declining maybe you look at that deal and wonder where they'd be without that thing on the books. But it's too early to even say that tbh.