I had surgery for an inguinal hernia three weeks ago and my left testicle has swollen badly over the two days is it serious?

Possibly. It is not uncommon for some swelling to get into the scrotum after inguinal hernia surgery. It is unusual, though, for there to be a change 3 weeks after the operation. You should be checked by your surgeon if this has not resolved.
No. The swelling happens when the venous drainage from the hernia is compromised by swelling after surgery.

Related Questions

10 days post opp on double left inguinal hernia, my left testicle is three times its normal size and the tubes swollen and painful is this normal?

Sometimes. Swelling after hernia surgery commonly affects the cord structures to the testicle ; the testicle itself. Bruising can be seen too. I cannot tell if the swelling that you describe is normal for you without an exam. Call your surgeon ; discuss with them as to what you should do. Read more...