I accidentally took an 875mg of amoxicillin 3 hours before i was supposed to.. does this count as an overdose and if so what do i do?

You will be fine: No, this is not a concern... The good thing about amoxicillin is it has a high 'therapeutic index' - which means that you can go higher on the dose without significant harm. I would try to stick to the recommended dosing intervals though, in order to distribute the medication throughout your body to best fight the infection. Hope you feel better soon!

No risk, no worry: Amoxicillin is an extremely safe drug. You could take 5 doses at once without harm, except maybe some cramping and diarrhea. Don't worry about it.