I think only one of them was involved. If all three were, I feel like someone would've cracked by now.It's possible that the other two DID tell LE everything they know and have turned against the third. It may be circumstantial enough, though, that there hasn't been an arrest.
It seems far-fetched that there hasn't been an arrest but it's happened before. 18-year-old Brookelyn Farthing disappeared in Madison County from a friend's house after a party. LE knows she was at the guy's house, knows he's the last one who probably saw her, and while she was there she even texted friends and family to come and get her claiming "I'm scared." The last person to see her claimed that he left the house for an hour and that he last saw her "sitting on the couch." The same couch that, later that morning, went up into literal flames and was completely destroyed.
Our LE aren't inept, but sometimes it's difficult to understand their decisions. Of course, in other cases that we thought were going cold, an arrest has come out of the blue and we've learned that they were watching and following the perp for months, trying to collect their evidence. So there's that.