Depends on who the third is matched up against -
If he's playing against top opposition and coming out even, I'm OK with it.
If he's opening up a top 6 line to play against weaker opposition, I'm OK with that too.
Looking back through the games to see his matchups and his line doesn't fare well against top competition. With more even line distribution by opponents he mostly breaks even or slightly behind his line's competition.
Against Vegas he was getting primarily 2nd/3rd line competition, the Marchessault-Karlsson line. Kind of tough to judge with Vegas because they've spread their talent around to have 3 roughly even lines. Kessel-Eichel, Stephenson-Stone, Marchessault-Karlsson.
He came away basically even with the only goal against (ignoring the EN GA) being the Pionk pass to Rittich who gave it away to Eichel.
It's an entirely different story against Colorado however, being primarily hard matched against Rantanen-Mackinnon-Lehkonen and Makar. Lowry's line got completely caved in metrics against them, unsurprisingly. Looking back it seems even more incredible that we started Lowry against their top line in OT intentionally because it appears to have been dumb luck that they even got a shot away
The Stars TOI matchups against Lowry were all over the place but Lowry came away mostly even even against top competition. The only player he might've struggled with was Jason Robertson but with only 4:05 TOI against Robertson it's tough to say
And finally against the Rangers it was more similar to the Avs game. Hard matchups against the Rangers top players 5 on 5 in Lafrenière-Trochek-Panarin and Trouba where Lowry's line got their shit kicked in.
In summary - Lowry's line probably shouldn't be matched against top lines if we can help it with last change ie. the Rangers game.