Personally, I find it helpful to try to follow the case as the jury is hearing it. As yet, Prosecution hasn't entered any evidence about the trio, or RA's initial statement.Maybe they did see a completely different man; it's theoretically possible. None of that cancels out the fact that Allen says he was there, and that he freely admits he was dressed like BG.
But Allen says he saw three females, and three females say they saw him. There's almost no chance that one of them would not have come forward early on. As I described in my example earlier, witness descriptions can be way off.
His clothing description was his first confession.
From the point of view of the Prosecution, and IMO the jury, the story they're telling is that police learned about BG from Libby's phone, and, after publizing the video, some people came forward claiming to have seen him hanging around the area.
IMO, from the point of view of the jury, they are being brought along the investigation step by step. To see, through LE eyes, how it lead them to (legitimately, the Prosecution hopes) arrest and prosecute RA.
Someone said, trials are won or lost by who tells the most believable story. It's not just duelling pieces of evidence.
JMO