Not a bad 3 years (with decent trades), but I don't see the value in trading for McDonagh (to not resign him) and dropping Gardiner to the 3rd pairing.
I feel that Dermott can take over for Gardiner on the second pairing going into next year.
I think the package dealt for McDonagh (or some combination) along with Gardiner, can land us a decent RHD with decent cap hit to play with Reilly; and, we should resign set RHD.

The Leafs really don't have the cap to sign Tavares next year, so it's really kinda of a moot point, unless they trade a Gardiner or Kadri without any money come back

Well, McD's the most acquire-able upgrade that'd help the defense right now... and he'd be under contract for the next season. That's 2 improved chances at going deep in the playoffs. Even if you don't re-sign him, there's value in that, I think. -- as for the 3rd pairing role for Gardiner: was assuming pretty equal distribution of ice time and situational mixing of pairs (as Babs has done lately). There'd be no weak spots on D. Which is, I think, what the team really needs to thrive...

More than a stud who fits a particular spot. What right-handed defensemen are available that are worth having? Does the proposed cost (throw in Gardiner) mean we're not moving the needle?

I honestly don't think our D is that bad, but we are weak on the right side.
I think a move can be made to bring in a RHD that is roughly equivalent to Gardiner in talent, but better defensively.
I don't think that player needs to be a rental (expiring contract), and Gardiner's cap hit is team friendly which could make it easier for a team that's getting close to the cap.
Gardiner and a 1st (2019?) could land a really good RHD.
I think our D-core would look pretty decent going into 2018-19, like this :
Reilly *RHD
Dermott Zaitsev
Hainsey Liljegren