I will have to submit this to the driver bug report as well but basically the new 15.5 Catalyst Beta enables Crossfire or both the above mentioned titles but TriFire is a dogs breakfast in terms of scaling, sure it's enabled but it introduces game killing microstutter in pCars and lower frame rates than a single GPU and the same goes for the Witcher 3.
No, it's not a CPU bottleneck before anyone suggests that, it's clocked in @ 4.7 on an i7 3770K and can easily handle similar titles which are CPU heavy in TriFire with no issues. Seems to be a driver issue.
I have tried all sorts of CF fixes and tweaks and nothing helps.
Please make sure the next update includes proper support for TriFire!