I'm not sure how many of those Marner would get you. Seth Jones easily (and you could get more probably) and yes, I would be interested in him also. Put him on this Leafs team and I think they are much better than they are now (depending on what you need to trade to get him).
McAvoy and Dahlin and out of the range of possibilities I'm afraid. McAvoy is a fixture in Boston, I don't think he is going anywhere for almost anyone.
And Dahlin? As I said before I live in Niagara County, NY (I can see downtown Toronto across the lake but technically I'm in Sabres territory so I get all the Sabres talk all the time) and unless something changes, he isn't going anywhere at anytime. Most view him as a future Norris winner, and think by the end of his career he might be in the top 5 Sabres players ever and potentially in the discussion of top 10 D-men in the history of the league. Will he be that? Who knows, but that is what many in WNY think of him.
I do think the single biggest thing this Leafs team needs, more than even goaltending, is a quality, 1st pair D-man who is between 23-30 years old and can give you over 20 minutes per game every night.