Please tell me, could implantation bleeding occur 3 weeks after intercourse, or is that too late?

Too late: Implantation is when the conceptus (from union of egg & sperm) attach to the wall of the uterus. Bleeding most often dose not occur with implantation. If there is bleeding the amount is small. It may be small rust or brown colored drops or light pink swirled in white d/C. It usually occurs about ten days after ovulation. There is more blood w period. Take a HPT if you miss your period.