//It does seem a lot for three hours work//
How much of it was for the work and how much was for materials?
If being charged £600 for a days labour then that was excessive too so I suspect it includes parts.
When you are on a job there are a lot of overheads that are static. You have to get there and back, go collect the materials, do the paperwork and accounts, pay for insurance advertising and your van and tools.
Probably find the hourly rate is actually minimal.