Are you matched with 4 player squad as a 3 player squad?

From my experience:

Solo: Everyone is by themselves (given).
Duo: You can play alone, or be matched up with a teammate.
Squad: You can play alone, but unless you turn off auto matching, you'll be paired with upwards of 3 others, ideally creating a full squad of 4.

From the FAQ and Wiki:

"What type of servers are there? | A: Solo, Duo, SQUADS (team up in groups of 2, 3 or 4 players, or if you prefer, you can still play solo and take on everyone alone)"

"Each game modes (Solo, Duo, Squad, etc) will be matched independently (For example, Duos cannot be matched with Solo queue)."

Note: Auto matching is found next to the queue type you intend to enter under 'PLAY'.

You can be matched against teams consisting of 1 - 4 people.

The only modes where you will be matched with even numbers is in Solo.

It's possible to play in Squad/Duo mode on your own if you turn off the auto matching feature.