How can I get my third floor radiators working?
No need to panic, it's probably pretty simple. It sounds like your system is full of air and just needs to be bled. You've probably bled all the pressure out of the system, so you'll need to re-pressurize it, then bleed the radiators again, then pressurize again, bleed again, until all the air is gone. 

Look at your boiler, there should be a pressure gauge on it. It should have a green arc, and maybe a yellow ark before having a red ark. Most likely green will go up to 2 bar(29 psi). Right now the needle is probably on the low end of the scale. 

You'll need to find your pressurization valve, and open it until you get some pressure. The pressurization valve is between your home's water supply and the boiler, all you need to do is open it until you get about 1.2 to 1.5 bar and then close it. If you cannot find the valve see if you can find the manual, or just ask a neighbor who seems handy. It's very simple and you shouldn't need to pay anyone to do it. 

Be careful not to over-pressurize the system, turn off the pressurization valve before the needle goes into the red.