Believe it or not this was among the first tricks that I learnt, I saw myself do it in a dream & I just had to learn it. The Jon Relf Cross is exactly the same as the Rainbow Cross except that the two crossing balls are thrown to & caught Behind the back.
Start with one ball in each hand. Hold your arms by your sides with your elbows in & your forearms pointing forwards. Simultaneously throw both balls up & inwards so that they cross on the way up & then peak directly above your head. Watch the balls for as long as you can then reach behind you & catch both balls behind your back. The throws are very important, both balls must dissappear behind the back otherwise it just looks like you are catching the balls out to the side. however, if the balls are too close together when they fall behind your back they will be very difficult to catch as your hands will get in the way of each other.
Remember to use the 'dead spider' hand position to make the catches, spread your fingers out as wide as possible & point them up slightly to create a larger catching area & form a cage to stop the ball falling out of your hand. This will reduce the chances of missing the balls altogether. Only slightly, but every little bit helps.
Practise with just two balls for a few thousand attempts. Then take all three balls & start juggling. Throw one ball straight up in the air about a foot above your head. As it peaks, throw the two remaining balls up & behind you just as you have been practising. Catch & throw the first ball & then catch the other two descending balls behind the back. Bring them back round to the front & resume juggling. Simple.