Thanks for letting us know your progress.
Are you sure its only running on 3? ( which would mean a fault on the ignition or injector of just that one cylinder)
or is it just low tickover? have you tried resetting the Ecu on the red switch and restarting.
I have this problem in cold damp conditions, Ecu suffers from a fuelling issue, in my case overfueling caused by not firing first go.
Yours might be underfuelling?
My normal tickover is 1000rpm when warm ( a distributor and coil engine)
(your year handbook says 875 +or- 50 rpm a wasted spark engine )
