You can sit the PPL exams at your local flying school. They will give you a license application form then just get the chief instructor to sign/stamp it for each exam you pass. Then take this with you to OBA.
I took two before I left, but if I could do it again I would definitely do them all. I spent the first week locked up doing the remaining 5 exams which made it difficult to concentrate on the flying(which is very intensive and requires your full attention if you want to get it done quickly). It will make your life a lot easier if you do them before you go. Especially Air law(you need this to fly solo) and Nav & Met(you need these two before you can do the solo x-country). And not to mention the fact it will free up your evenings to relax and have some beers which I found was essential after a hard days flying.
Also I hope you have made provisions to stay longer than three weeks if necessary, although it's possible, it's unlikely you will be done in this time. I lost a week to weather so I was out there for 5 weeks in all.
The exams are easy to pass though. I used the Jeremy Pratt books, if you do the practice questions book you will pass the exams.