OK, so I had some time to analyze the hand history provided by OP.

During the hands when gd1990, caroline93, and MichaelDE are playing by themselves, there ZERO showdowns. So not 1 single showdown in the first 43 hands. That's a bit odd.

The first showdown happens in hand 65, or #242704944, after valentinos77 and AshPK have joined in. gd1990 wins the pot.

The second showdown happens in hand 70, or #242712360. MichaelDE wins the pot.

The third happens in hand 79, or #242712484. gd1990 wins the pot (the one in this screenshot).

So you can see that showdowns only started happening after other players had joined the table. The longest streak of no-showdown, across all 867 hands, is by far the first 65 hands; 43 of which were only played with the 3 players suspected of collusion.

While other players were playing:

Showdowns between gd1990 and caroline93: 5

Showdowns between gd1990 and Michael DE: 11

Showdowns between MichaelDE and caroline93: 1

Across 700+ hands played together, there was only 1 showdown between these two players... That's pretty anomalous, don't you think?