I've been taking amoxicillin 500mg + clavulanic 125mg twice a day for pericoronitis for 3 days. It's still painful. Do I need to take another course?

Needs intervention. Need to see oral surgeon now to have inflamed infected tissue removed. Surgeon can decide on need for more antibiotics after he has examined you.
Pericoronitis. No you need to get to the oral surgeon asap all a 2nd course of ab's will do is to culture bacteria resistant to Amoxicillin and then it wont work at all dont fool with this go tomarrow.
Continue treatment. 3 days is a short time to treat an infection, and a longer duration of antibiotic therapy may be indicated - along with the treatment plan your dentist recommends.