Recommendations of a company to install radiators

Knowing where you are might help.

Have had good results by picking local plumbers out of Yellow Pages.

What kind of radiator are you talking about?
Assuming that it is a normal central heating radiator fired by gas or mazout it is hard to understand how it might not work. All it is is a container for water which has been heated and pumped round the system. So long as the pump operates correctly (are other radiators getting heat?) I would be looking at what the problem might be before setting out to replace them.
Perhaps describing the symptoms might get you some advice.

KASSEISTAMPER might not be far from the truth there. Maybe all you need to do is bleed the radiators and let some more water into the system.

"Flat" can mean shared heating system as well. Our plumber said new radiators are smaller, but certainly not better in terms of heating efficiency.

If the radiators have thermostats it might be the thermostats - my mum in the UK just had three thermostats replaced on radiators that weren't working properly.

Agree with the others, you probably have a pump / circulation or thermostat problem. Radiators don't normally "break".

Also, if you're in a flat, you should talk to the other people in the building to see if there is potentially a problem with the heating in the building as a whole, rather than just your three radiators.

Rather than judging what the issue is when you don’t know what’s been tried or the system manybesomeone could suggest a good plumber or chauffagist