Pretty sure that Half Life 3 would run Source 2.
Obviously when they announce a new version of source they always release one of their prime games with the release of a new engine.
In this case Half Life 3.
When you think about it it actually makes sense for Half Life 3 to take a long time to come out, do you remember how long it took for
Team Fortress 2 to come out? Team fortress 2 was in for 9 years of Development, including an engine switch from GoldSource to Source Engine.
Their taking their time on it for a reason.
