I had unprotected sex with a girl and after three days had sore tonsils and white patches on one tonsil only with redness in my throat. STD??

Possible... Probably should have a doc look at it and take a culture of the sores. AND, if you want to avoid any worry of STD's or accidental parenting in the future, wear a condom---it's worth it!! Don't forget, some presents keep on giving. .
Should see your MD. It may be a tonsil infection but it is probably better for you to be checked. A culture could be taken and you may need to be treated.
Not an STD. No. Strep throat or viral pharyngitis after a sexual encounter is a coincidence. It is not a sexually transmitted disease. There is no STD that presents like this.

Related Questions

Can an STD in your throat cause one side to be sore, one tonsil mildly swollen, with redness on the crown of your throat, and itchy?

STDs. There is only one STD that causes pain in the throat and that would be Herpes. You are not in any way describing an STD by the symptoms you list. Read more...
Sore throat. It could be especially if there is alot of pus but it is much more likely to be from another bacteria. If it is on one side it could be an abscess and that needs drainage since antibiotics are less likely to work. All and all a trip to the MD is in order. Read more...