MPS and Sundqvist have been on the ice for 7 combined ES GF and 8 against. According to naturalstattrick, they've been on the ice for 63 combined high danger ES scoring chances for and 44 against. This is in spite of having the least favorable offensive usage on the team. (Note that these numbers are additive among individuals, so if they were out there together for 1 goal for on the ice, it's being counted as 2 goals for in these numbers even though the impact for the team was only 1 goal. Point being that the on-ice impact for the team is being overstated based on these numbers.)
Jaskin's numbers are even better, but he's spent more time in the top 6 and his usage has been a bit more favorable, so they may be skewed a bit by that.
Collectively, they aren't contributing much offensively, but they definitely aren't hurting the team, either.
Personally, I don't see anything to complain about here, especially given the circumstances. The quality of the special teams play is holding back this team far more than the third line is. The difference between their current efficiency and the 10th best efficiency in both categories is more than 5 net goals already. Compared to the production we had from those units last year, it's almost 7 goals in just 20 games...a pace for approximately a net swing of approximately 28 goals over the course of an 82 game season.
You're going to have a hell of a time trying to bolster a 3rd line to the point where it's almost 30 net goals better than it currently is. Can anyone even name a team with a 3rd line that dominates its competition to that degree? I doubt it.
I don't know what the best teams regularly get from their third lines, but I feel like a team is doing pretty good if its third line can eke out a net gain of 5-10 goals at ES over the course of a season. There were 16 teams last year that had a positive goal differential, and the average positive differential for those teams was +30.5 goals. A third line contributing 5-10 net goals would represent 17-33% of that total, which seems high (especially at the upper end) considering that there are three other lines and two special teams that also factor into that mix.