The front three scored four of United's five goals in the win over Bournemouth on Saturday and have now combined for 55 goals this season -- more than Sadio Mane, Roberto Firmino and Mohamed Salah at champions Liverpool.
"We have to improve the ones we have here, think we've seen improvement from Mason, Martial and Rashford and they will keep on improving but we will never stand still always on the lookout if anything is possible," said Solskjaer after the 5-2 win over Bournemouth at Old Trafford.
"You can't think we have cracked it like Gary [Neville] mentioned last week. The emphasis is on improving the team all the time."
Greenwood scored twice either side of half-time to add to the goal he got in the 3-0 win over Brighton on Tuesday.
The 18-year-old has got 15 in his first season in senior football, only five behind Rashford and Martial.
"We will always come back to his finishing and the goals were excellent," said Solskjaer. "His general overall play is improving, his fitness is improving, his understanding of his position is improving more and more and we're delighted with the goals he scores, right foot and left foot."