It's rather odd booking, because if you want to be technical, neither Reigns or Brock should be there considering they were both eliminated, Ambrose being their makes sense, he's the runner up after all. I suppose a case could be made for Reigns, if you want to get technically, since he was "taken out", but he still came back and was eliminated.
The situation should have probably been Ambrose/HHH for the title, and then have a #1 Contenders between Reigns/Brock. But if they mean to make both of them look strong walking out, there needs to be the third man in Ambrose fits the bill and makes the most sense in the match.
Hard to say though, something could happen in the upcoming weeks. Generally the whole of Brock/Bray was treated oddly the moment that Brock simply walked away from the ring rather than going in the ring and flat-out destroying everyone.
Generally though, the position calls for it. If the idea is to place obstacles in front of Reigns, there isn't much better an option than Brock, and Ambrose's performance in the Rumble and connection to Reigns is a good enough reason to include him as well. With the main event scene at the moment, needing some work (which if all goes well, they should have a pretty good scene once WrestleMania passes), it's probably best to throw Brock into the mix.
Hopefully though, they do something before it happens (if it does), even if it means hinting at something happening, rather than just having it happen.