Yes, you can but its unlikely. The fact that you are three weeks in means it is less likely, MOST women are likely to get pregnant at day 14. Also, although the guy didnt ejaculate but there is a chance sperm is in the precum. the fact that he didnt fully enter means there is further for the sperm to travel, meaning its less likely.
In answer to your question;
Yes you can get pregnant this way, BUT its fairly unlikely. If you are worried then you should take the MAP.