McGill’s 901/906: Largs - GlasgowCan you think of a bus route in the UK that serves at least 3 (or more) counties?
An interesting extension might be short routes which serve three counties.
A lot depends on how you define county, and how you define serves.
Take the X5, for example. This route runs from Oxford to Bedford (traditionally it continued to Cambridge, but it has been curtailed).
This certainly serves Oxfordshire, Buckinghamshire, and Bedfordshire. But Milton Keynes is technically separate from the rest of Bucks, and so you could argue this is a fourth. The route also runs through Northamptonshire (on the bypass around Old Stratford) without stopping, which could be viewed as a fifth.
I'll try to give some unambiguous ones which spring to mind, using the current ceremonial counties.
Falcon: Bristol, Somerset, Devon
X18 (Evesham - Coventry): Worcestershire, Warwickshire, West Midlands
X10 (Newcastle - Middlesbrough): Tyne and Wear, County Durham, North Yorkshire
685: Tyne and Wear, Northumberland, Cumbria
The new X57 service from Manchester - Sheffield: South Yorkshire, Derbyshire, Greater Manchester
An interesting extension might be short routes which serve three counties. I'll start the bidding with the X3 from Salisbury to Bournemouth - not a very long route really, yet squeezes in three by going via Ringwood, Hampshire. Are there any shorter?