Is it possible to over feed my 3 week old baby? He's breastfeeding, not bottle feeding.

Possible. Breast fed babies usually stop sucking when they feel full. If your baby is spitting up a lot, that could mean he is taking more than he needs, but at this age, don't worry about overfeeding. The baby needs lots of calories for growth.
Over feeding. Babies has tendency to have sucking reflex. So it is easily for these babies to keep nursing until they over feed. Then they tend to spit up alot.