You should note that Chelsea is a team that has performed well in the past, but PSG has only been commenting well this season. In the past, PSG's performance was very bad, they lost most of the matches, but this season they have roared and entered the final with a bang. However, Chelsea could not control the game in any way, seeing such a gap on the scoreboard, PSG players were very nervous.
And as time went on, victory was getting away from the PSG team because it was impossible to score three goals in the last minute. And in the end, PSG was not able to score a single goal, and the attacks were very weak for PSG. Because Chelsea's defenders prevented the attack before it reached them.
In short, you can say that Chelsea was a more experienced team as compared to PSG, and this thing came to their advantage. PSG weren't comfortable and were feeling the pressure of the final. That is the reason that Chelsea took early advantage in this game and quickly scored three goals in the first half of the match, making the match totally under their control. Palmer scored two back to back goals and gave early advantage to his team.
PSG had such a big semi final game where they beat the giant Real Madrid by 4-0, but in this final they completely collapsed.