Try turning all the radiators except one of the cold ones OFF and make sure the last one is fully ON. All the pressure will go to the dead radiator and might just get it going. Repeat for the other two dead ones.
Incidentally, I never turn radiators fully off. If you want one off torn it off and then back 1/4 turn; that way it won't stick. It will only allow a very small flow of water, so no great loss.