I kinda agree with sub here.
£1 per hour for the first 3 hours
This implies that (as an example) if you stayed there for five hours, you'd be charged £3 for the first three hours and then whatever was the cost of the remaining two hours.
The problem here is that it sounds like the parking company don't have a particularly strong grasp of English. Their advert would work if there was an hourly rate eg £2/hr. That way, parking for five hours should cost £7, not £10.
The sign should have said something along the lines of:
£1 per hour for the first 3 hours for up to 3 hours
or
£1 per hour if parked for 3 hours or less
Either way, there should have been a disclaimer stating that any stay over 3 hours would mean the whole stay would have been parked at the normal rate.