The Rainbow Cross is closely related to Columns (you don't have to be able to do columns for the Rainbow Cross but it does help) in that it involves throwing one ball & then two balls simultaneously, only this time you vary the throws of the outside balls rather than the centre ball. This trick is sometimes called the umbrella.
Start with two balls & hold one in each hand with your arms by your sides pointing forwards & outwards so that your elbows almost touch your body & your hands are over shoulder width apart. Look up & pick a spot above your head, this will be the Crossing Point. Looking directly at that spot at all times throw both balls at the same time with outside scoop so that they both peak & pass each other at the spot you are staring at. Because the balls cross you catch each one in the opposite hand that threw it.
If the balls both peak at exactly the same point in the air they will bounce off each other. You can try aiming for a hit & they will more than likely miss, but with practice you will find that your accuracy improves & they do collide. Try throwing one ball slightly higher so that the two balls pass above & below each other or, throw the pair with one hand slightly in front of the other so that one ball passes behind the other. Try to get the balls to pass as close as possible for the best effect.
When you can consistently cross the balls over & catch them, put the move into a pattern. Performing the move in Columns is simple enough, when it comes to making a regular pair of vertical throws, cross them over instead. You can easily do a Rainbow Cross straight from a Cascade & go back into the cascade. In the same way as you go from a cascade into columns, throw one ball straight up from the centre of your body to the crossing point, as it peaks throw the other two balls. Catch the centre ball as the pair cross & throw it straight back up through the centre, catch the other two, wait for the third to come down then resume juggling.
When you are happy with that try moving the centre ball around as described in the Columns tutorial. Or throw a Rainbow Cross up into an Overhead Cascade. You may even like to try the Jon Relf Cross.