He put the third line back together because they "had been great all season driving play down the ice and getting the team going", which is exactly what they did. They began driving the Sens backwards and recapturing the momentum, allowing the Leafs to establish sustained, shift after shift pressure in the offensive zone.
The third line has not only been a shutdown line all season, but they have also been a handful in the offensive zone. All three of them are big bodies that can protect the puck down low on cycles but also have the speed to lose defenders and create chances off the wall and rush. In their 5 minutes together against the Sens, they had 4 defensive zone faceoffs, 2 offensive zone draws and had a 100% corsi and expected goals for percentage. Talk about dominant.
The line change may have just been for the game but it wouldn't be surprising if Keefe keeps the line together for tomorrow's game against the tight checking Islanders back in Toronto.