Eddie's point is absolutely correct - it is not possible for these owners to have rights to park there.
To claim ownership of the land, an individual has to demonstreate factual possession of the land and 'maintain' it for at least twelve years, after which the start of a process to claim adverse possession can occur by application to the Land Registry. Often having 'factual possession' involves fencing it off, as that excludes others.
Because there are 3 different local owners vying for the same land, it becomes more complicated, but it is still possible. You would all have to work in a pact, know and agree what you are doing together, then apply together for joint ownership of that land after the appropriate time period.
You are going to have to come clean about your scheme and explain it to the other two - you will never be able to demonstrate adverse possession alone whilst they have been parking there.
The LR Practice Guide Sheet that explains more is this one - but it is complicated and in legal terminology, because it is intended for use by conveyancers.
http://www.landregist...ides/practice-guide-5