In the studio, the default time signature is 4/4. You can change this to 3/4 if you click the "Grid settings" cog wheel in the upper right corner of the main editing window (1) and select it under "Time signature" (2). The difference between the two is that in 3/4, you count three beats per bar (3), and in 4/4, you count four beats per bar (4).
Please note that if you change the time signature this will only affect the grid. It will not change the position or length of individual regions or loops. If you want to learn more about the metronome and how to set the tempo of your project in BPM (beats per minute), click here.
(1) Grid settings
(2) Time signature
(3) Time signature set to 3/4 (count 1, 2, 3 every bar)
(4) Time signature set to 4/4 (count 1, 2, 3, 4 every bar)