Halo 3, Master Chief's debut on the Xbox 360, is next in line to be added to the PC version of Halo: The Master Chief Collection, and it sounds like we won't have too long to wait. As 343 Industries explains over on the MCC development blog, public testing is expected to begin in the first half of June.
"Halo 3 is next up on the menu for coming to MCC on PC and we are already underway with early stages of flighting in Ring 1. Next week, we aim to get the completely paired down flight build into our partners hands, which is much sooner than usual. Our goal is to have the Halo 3 public flight kicked off in the first half of June. We can’t wait to get the community involved!"
Note that 'flighting' means 'testing', so if all goes well, the public should be able to get its mitts on Halo 3 in early June. There's no word on what this testing will include yet, but, presuming it's similar to the Halo 2 public testing period, it's likely to offer a sampling of campaign missions and multiplayer games, with the contents changing day by day.
Early this month, 343 revealed the first screenshots of the PC Halo 3 and ODST. You can have a butcher's at them here.