Why 3 Downstairs Radiators are cold?After new Combi Bioiler?

first, make sure all radiator valves are open, one rad valve on the rad you can turn by hand to open, anti clockwise. the other valve you would need pliers to turn, then turn OFF all the radiator valves on rads that are heating up, start the boiler up for central heating, now,,, the heating has to circulate, so the hot water has to go somewhere so it willgo to the radiators with the valves open, when they heat up give them about 15mins then open the other radiators and they should start heating up. it sounds like what has happened is air has trapped in the pipework somehow not the radiatos, so if they all heat up, turn of the boiler and re/pressure the boiler as it will have vented the air and lost some pressure, good luck.