The way I see it, PSG have already won the league double this season. They have won the league and they're in the final against Reims. Except Reims turns does an absolutely impossible thing by winning PSG in the Coupe de France final, I don't see what will stop PSG from winning the treble.
The UCL is a whole different thing, they still have to get past Arsenal before they can win the final. As much as they are one of the favourites to win the Champions League, it's still a difficult journey for them.
Barcelona can still win the treble, but they can still lose everything as well, despite the impressive season they're having. They've not secured any of the trophies yet, so they still have a lot to do.
The same goes for Inter. They can lose the league to Napoli, and if they aren't careful, they can lose the semi-final Coppa Italia against Milan. Still a lot of work to do.
This is the most difficult period to maintain consistency. If successful, teams like Inter Milan and Barcelona will achieve a treble winner, and if they fail, they could lose everything. Arsenal will clearly make the Champions League their main target, as they are now only aiming to finish as Premier League runnerups. The situation is more favorable for PSG, as they no longer have anything to worry about in domestic competitions, so Luis Enrique will pour all his energy and focus into the Champions League.
Besides that, in this crucial period, teams that avoid injuries will have a greater chance of winning trophies.