That's a hot water storage cylinder not a combi.
The upper 2 are central heating not tap water. Lower one would have to guess.
Look up "airlock" or "Air Lock".
If you have a mixer tap in the kitchen and are fairly strong-handed, turn the HW tap on, then the Cold, with the middle of the palm of your hand HARD over the end of the tap. Cold should push its way up the HW side to fill the pipe with water. Take your time. When you let go, the HW should run.
Why did you turn the taps off?
And don't leave then right at the very end of the Open end or they'll jam - go right half a turn.
