PSG have the best chance to achieve both league trophies this season but the champions league is still a major doubt. While Inter milan have a good chance at the champions league, their league title race is a tight one as they have safe strong competition from Napoli. In my opinion, Barcelona have better chance at winning a treble this season.
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.