And many players aren't happy about Trio's removal, it seems. The change comes as part of the third season of Call of Duty: Modern Warfare, which introduces the new Quads mode to the Warzone experience. 

Quads is effectively the same mode as Trios, but in teams of four instead of three, as the name suggests. As such the change has irked a number of fans, who were used to playing in squads of 3 and are now trying to find a new player to integrate into their team.

As for why trios was removed instead of retained instead of a separate mode, that isn't entirely clear. Activision did state their motivation for the quads mode, but not why trios was canned;

Season Three of Warzone brought in Quads, a twist on the game’s standard Battle Royale by way of adding an extra squadmate to each fireteam. A fourth squad member may not seem like much, unless you consider that’s much more firepower being added to each and every team, and that a one-on-four situation is much scarier than a 1v3. Having an additional Operator will also make certain Contracts easier, such as Scavenger Contracts, where your team can split up slightly to capture Resupply Boxes scattered around the map.

Players took to the CODWarzone subreddit to complain about the changes, with some players stating the decision as "the worst ever", with the top voted post asking for answers as to why trios was removed.

One theory is that it would create issues with servers and queues, splitting the playerbase, but it does seem like an odd move to totally pull the rug out from under the feet of those who were enjoying the mode. Activision has yet to state anything further than the above, but we'll be sure to report on any updates.