My 3 year old son's testes haven't dropped. should i be worried?

Urologist to check: Term babies can have an undescended testicle. There is usually something wrong in the undescended testicle, related to why it did not descend. It may be smaller and only partly functional. It also has a higher chance of cancer. However, preemies with undescended testicles probably have normal testicles but are just born before testicles are supposed to go into the scrotum in the 7th-8th months.

Yes: I assume from your post you mean the testis are never in the sac. They should have come down before birth & delay up to 6 mo is sometimes seen. After 1 yr, any delay can have an impact on later fertility & increase risk of testicular cancer in their lifetime. Have the kid evaluated within the next month. They need to find the testis & move them into proper position if possible.