You are right to think its a 3 port valve, the fact its only 3 yrs old don,t matter, check the wiring is in order.
Should be a lever on the valve that enables you to open it manually to get hot water if it packs up.
Sounds like problem lies with the salus thermostat, someone nearby is controlling your c/h with their thermostat.
Reconfigure the pins in the wall stat to change frequency (go to instructions).
If problem persists get your GSI to disconnect the thermostat wiring to the boiler and re connect link...
Yep WB plastic components are a joke, with the flow sensor, diverter valve and hydro block all housed in plastic and they all eventually fail with fractures and seal wear.
Not to mention nearly all the water carrying connections are push and clip fit,
But I,m not complaining repairing them...
Or it could be a bath hot outlet is 22mm and a basin 15mm, with the outside mains flow much cooler this time of year and a low kw combi would produce luke warm water from hot outlet.
Worcester bosch have a very good marketing structure and price tag, other than that nothing special at all.
Some research on a Intergas boiler might be worthwhile.
Seen this many times with Vaillant ecotec, one of two things usually, a leaking diverter valve (internal seals)
Or internal flow or return pipe seals leaking, the connections being push and clip with an O ring seals.
Put a hose on the c/h drain cock (use a jubilee clip to secure hose) open drain cock and back fill system using mains water, 10 seconds should be enough if its a airlock.