Would going for a full body massage 3 weeks after a septoplasty cause damage?

Please ask your surgeon for their advice, because they know your nose better than anyone here. Having said that, if your face is down in the open hole during a massage, that shouldn't disturb the surgery site itself. There will be some blood pooling in the nose which leads to congestion during a massage. This should be temporary, and I don't think would disturb your healing septum three weeks out from surgery.