I think this is not really difficult trigonometry. Suppose we want the robot to move in a forward direction? We need to use the two front motors. (I tried to sketch this out in the pictures, I will now explain all this scribble). Due to the fact that the motors are not at right angles to the axis we need, therefore, we need to feed the motors a little higher speed so that it travels at the axial speed we need. To calculate how much you need to increase the speed for the motors so that the axial speed is necessary for us, we need trigonometry.
Imagine that we need to move forward at an axial speed of 10. What speed must be applied to the left and right motors so that the entire robot moves forward at a speed of 10? Let's find out. X is the required axial speed. The front wheels are off-axis by 30 degrees. The side is adjacent, so we take the cos of 30 degrees. We get 10 / cos(30) is approximately 11.54. But we have two motors, so we just halve the speed and feed the motors. (I do not take into account the direction of rotation here).
Now it's more interesting how to move to the sides? In this case, all motors must move in the same axial direction. Let's start with the back motor. It stands parallel to the axis we need, so we don't need to change its speed. Well, just split it in half, because the top and bottom parts of the robot must move at the same speed.
Now the front motors. They are located at an angle of 60 degrees to the desired axis. The side is again adjacent, we take the cosine. Next, we take half of the required speed, divide it by cosine 60, then divide it in half again, because there are two motors in front and that's it. Well, an example. If we want to move at a speed of 10 to the side, then we need to apply speed 5 to the back motor, and 2.5 to each of the front ones.
So that's why there is a cos(60).
