The roll axis can be taken as the x-axis and is straight ahead.

The red ball that represents it is hidden behind the white ball.

The pitch axis is the y-axis, out on the left wing. It is represented by the blue ball.

The yaw axis is the z-axis, upwards, represented by the yellow ball.

At the start, increasing roll rotates us clockwise - we are looking outwards along x.

Increasing pitch puts the nose down.

Increasing yaw turns us to the left.

But when the rotations are combined, the effect depends on the order of matrix multiplication.

Here roll rotates about the 'red' axis,

yaw rotates about 'straight up',

But the pitch axis does not roll with the aircraft.