Should I pay my ticket for beijing shuttle bus before I get in or can I pay to the driver?

You need to pay before you get in.
The ticket office in T3 is located in the exit of Zone A, opposite of the exit of Zone C onF2; next to Gate5, 7&11 on F1. Take your passport there and you can buy it. Shuttle Bus in T3 is located near the Departure Level.
There are many bank offices you can change your money:
Hang Seng Bank in the public area of International Arrival Hall, F2, T3-C (west of Exit B): 24 hours open
Bank of China in the east of the departure check-in hall,F4, T3-C: 9:00 – 17:00
China Construction Bank in the west of the departure check-in hall, F4, T3-C: 9:00 – 17:00
Agricultural Bank of China in the public area of International Arrival Hall, F2, T3-C (west of Exit B): 9:00 – 17:00