Greenwood started the scoring against Partizan Belgrade midway through the first half before two other goals were scored at first-half goals at Old Trafford for the first time this season.
United cruise past Partizan to qualify
Rashford completed the victory with a rasping left-footed drive early in the second half as United qualified from Group L with two games to spare.
"I'm very pleased with all of those finishes, all three goals were high class," Solskjaer said.
"Of course, we should have had a few more, but the first one settled us down. Mason showed great composure; with the goalkeeper going down, he changed his mind and I was very happy with that.
"Anthony's was probably the skill of the night, and then I was very pleased Marcus scored with his left foot. He'll need more goals on his left because he is in that position often and he can go both ways.
"Marcus got in some great positions, made some great runs onto some great passes, the goalkeeper made some saves and the other times he didn't hit the target, but the goal he scored will give him loads of confidence.