There's a pretty weird bug in Apex Legends right now that's accidentally putting players into trios or duos matches without any teammates, which, as you can imagine, is leading to a degree of frustration among players.
Even with the "Fill Teammates" box checked, the game seems to be dropping players into games without any teammates, even in Arena matches. Respawn has stated on Reddit they're aware of thee bug, but haven't yet said when it might be fixed; "God I hate this bug. We shipped some fixes with the season update that we thought would get it. Just gonna have to take another shot at it." said comms director Ryan Rigley.
The issue has been around since March's update that introduced the "Fill Teammates" checkbox but seemingly has become an increasing nuisance. Players aren't entirely sure what causes it, but there are some theories on how to avoid the issue, with one Redditor, Uncredited1, stating that it seems to happen on a pattern; "I only play trios. The first three or four games, I am alone or with one teammate. After that, I get a full squad for about five or six games. Then the cycle repeats. It fluctuates, good days and bad, but it does not seem to matter what time of day it is."
But until there's an official fix, it seems that you're just going to get unlucky from time to time. Hopefully, a fix is rolled out soon.