Are you able to stay longer than 3 hours or is that the limit? If not, then the answer is just to pay to stay a bit longer but if you cannot do this, then I think it is a bit harsh given that it would be quite easy to go over the limit.
I know that the general advice given in these situations is to ignore the ticket, but can I just defend this from the position of those responsible for maintaining the car parks?
We ourselves run a cafe business at which we have parking for customers. When we took over the premises it was just a wasteland at the rear and we spent many thousands of pounds turning it into a parking area with space for at least 14 cars. As soon as we finished the parking area, people living in the flats around the premises started to use this area as their own private parking. We asked them to stop leaving their cars but were ignored, so we had to get a company in to patrol the area for us and give tickets to those that abuse the parking. Of course we also have an ongoing cost to maintain the car park as well, to ensure that it remains in a good state for our customers.
There often is a good genuine reason why businesses put controls in to stop people abusing the car parks or they would not risk alienating customers, there is a cost to upkeep this area, but some people seem to forget this!