Only Direct flights into the United States require 3 hours (or more) — due to US Customs & Immigration PreClearance.
Two hours prior to scheduled departure time SHOULD be sufficient for all other International flights.
That said, it could take as long as an hour to return the rental, finalize the paperwork and make it to the check in counter.
We’re it ME, I would allow at least TWO hours for the drive ( Google Maps says 1 hour 31 minutes) — and I would probably add a little extra!
Don’t forget paying the M50 Barrier Free Toll — It CAN be prepaid at any pay point shop — or online — but I would want to have a physical receipt — just in case the rental company asks for it.