How do i add pressure to my baxi 80 boiler?

The pressure on my boiler is showing 0.3, there are 3 black taps under the boiler, one to right which stands alone and then 2 which stand together, i have tried turning them all and when i turn the one furthest to the left the pressure goes to 0 when i turn the others nothing seems to happen. Do I need a filling loop or am I just doing something wrong?