I have 3 testicles. is this normal for a guy to have three?

Remotely possible: Having 3 or 4 testicles is a very rare abnormality, but does occur. Spermatoceles, or cysts of head of epididymis are the most common finding when men or boys think they have a third testicle. I have seen two boys each with proven 3 testicles & one with 4, all normal by ultrasound, & thus requiring no treatment. This is after examining about 200,000 boys & men during my career as a urologist.

Very unusual: This is very uncommon, and I would guess that one of the three things in your scrotum isn't a testicle but a cyst, hydrocele, or other abnormal mass. Assuming this is unchanged most of your adult life, probably it's nothing dangerous. Still, you should see a doctor soon to sort it out and advise whether diagnostic tests (like ultrasound) might be recommended. Good luck!