If I approach the water company and have a new supply put in will it
have to go on a water meter

That's up to them. However, you would be strongly advised to request one.
You are very likely to pay less on a meter in a gardenless flat, even with
long daily showers or baths.

or would the water co do this new supply for free ??

Sorry to disappoint. They use new water connections to buy the chairmen
their yachts.

I presume the 3/4" pipe is not big enough for the 3 flats given the
distance to flats etc:

Unfortanately, the replacement will not increase the static pressure when
others in the building are not using water. It will improve it when they
are, though. It should greatly increase the flow rate, though.

However, the low pressure could work to your advantage. The water company
must provide at least 9lpm and 1 bar pressure. It sounds like they are
failing to do this and might have to upgrade you for free. This could
actually involve upgrading street mains, if the pressure is that low.

Christian.