This is the same as the ball trick that shares its name: one hand juggles two, and the other hand throws the third club up and down in time with them. The trick looks best if all three clubs stay in columns, but it is also possible to use outward circles for the two in one hand pattern.
There are two timings that are commonly for the throwing of the lone club: in time with the inside column, or in time with the outside one.
At first it can be hard to match the height and spin of the two clubs that are thrown together, but as long as you can sustain the pattern the finer points will come eventually. A tip that might help is to concentrate on moving your whole forearms (rather than just your hands) together when making the parallel throws.
Instead of throwing a pair of clubs straight up and down try crossing the throws over, with one hand doing a normal double across to the other side, and the other throwing a double backcross.