We do not have a deep roster. The roster, especially the bottom 6, is the weakest link for this team. And when you combine that with Sullivan's biases, yes, of course we are going to see lineup issues in the third.
Sullivan basically double shifted Carter the majority of the Leafs game running Smith-Eller-Carter. That's not a line that's conducive to pushing offensive play. That is the "well, we think that is the Leafs get it in the defensive zone, this line gives us a better chance not to be scored on". And that's not a good strategy for this team. This team does not have the defensive system (or players) that can go into the defensive shell. To borrow a cheesy quote from Miracle and Kurt Russel - you don't defend them, you attack them. Carter is not the man that does that.
Now that said, while I might not agree, I can understand if Sullivan is iffy on the defensive prowness of Puustinen, Bemstrom, and Puljujarvi but to the point the poster was making - that forces them to rely heavily on older players which over the course of a season will certainly have a negative effect. That's on Dubas to give the team a better bottom 6 support staff. And not just one that is "defense only". If he could have constructed: Bemstrom-Eller-Puljujarvi as the 4th line, that is a line you should be able to put out when you're up by one with 10min or 5min left in the 3rd.
I noted in that Toronto game that I could recall when Smith-Eller-Carter had anything of substance going on in the game outside of surviving the shift but I DID notice Smith-Eller-Puustinen several times.