I've a swollen lymph node above my left collar bone 3 weeks, no other symptoms. Went to dr had a CBC and mono test. Everything came back normal. Next?

Time. Most enlarged lymph nodes are inflammatory in nature and not related to tumor. These nodes may be tender. Ultrasound is good tool for characterizing abnormal lymph nodes from normal lymph nodes. If enlarged node persists further investigation necessary. Doppler ultrasound helps distinguish by vascular pattern. Normal lymph nodes architecture usually different from nodes infiltrated by tumor.
Ignore it. Many guys your age have a few of these. If it's cancer, it would be stuck to the surrounding structures, or stony-hard, or become preposterously large, or be attended by fever / night sweats / weight loss.