"7+3" in the context of chemotherapy is an acronym for a chemotherapy regimen that is most often used today (as of 2014) as first-line induction therapy (to induce remission) in acute myelogenous leukemia, excluding the acute promyelocytic leukemia form, which is better treated with ATRA and/or arsenic trioxide and requires less chemotherapy (if requires it at all, which is not always the case).
The name "7+3" comes from the duration of chemotherapy course, which consists of<unk>  days of standard-dose cytarabine, and<unk>  days of an anthracycline antibiotic or an anthracenedione, most often daunorubicin (can be substituted for doxorubicin or idarubicin or mitoxantrone).
7+3 regimen
TIP regimen
RCD regimen
EOX regimen
ICE-R regimen
TCH regimen
DCEP regimen
FL regimen