First of all, what do your by-laws say? If they say no parking on common property without the written permission etc etc, how hard would it be for your neighbours to get written permission?
Secondly, this is something you should politely raise with your landlord and tell them you are being denied the full use of your garage by the other owners. If they refuse to do anything about it, ask for a reduction in rent.
If the landlord gets sniffy about it, point out this is a problem they are going to have with any tenant who expects full use of the garage. You are not the problem, it’s your selfish neighbours.
If they are using their garages for storage, so can’t park in them, point out that this is illegal under council laws. Otherwise basic respect for other people should prompt them to put their cars in the garages where they belong.
Re car washing – that is “standing” not parking – there is a big difference, not least that you are there to move the car if anyone needs to get past.