Ive learned over the years that not all things go to plan, so I will not speculate on what GTR3 will bring. Personally I think they (simbin) are relying on the history of their name to carry them, problem is they are a new simbin, not the old dev team from GTR2, and from all counts are still searching for key staff.
If SimBin can release their GTR3 title within a short period of ACC's release then they may take some of the market, but the longer it takes them to bring something playable to the pc screen the harder it will be to convince the die hard GT3 racer to migrate from ACC to GTR3.
I'm a fan of the WEC & endurance racing, love to watch it on the Teev, but where is the market for endurance sim racing?
There's very few endurance events held on a regular basis, like monthly, the Bathurst GT event run on the weekend is an example of how rare these awesome endurance events are held. I get the impression most sim racers prefer sprints over endurance racing, but I could be very wrong in this assumption
I guess in the end we sim-racers will decide what we like & dislike, but we will be spoilt for choice