This trick combines The Box & Swaps. It is far easier to master if you learn both tricks separately.
Begin with all three balls, two in one hand & one in the other. Throw the first ball from the hand holding two straight up to about 6 inches above your head, as you throw count, "throw, 2, catch". Look up & watch it as it peaks on the "2".
Next do the same again but as you count "2" when the first ball peaks throw a Swap underneath it & catch on the third beat as before. Don't look down when you throw the swap, fix your gaze on the high ball. You should be able to feel where your two hands are & you can judge where to catch the two swapped balls by feeling how you threw them. Make sure the horizontal throws are thrown firmly so that they travel straight into the opposite hand without rising or falling so that you don't have to noticeably move your arm to catch the ball. Throughout this trick, try to keep your arms as motionless as possible, make all throws with the wrist.
Practise this move starting with two balls in the left & two in the right.
Now do the same yet again, but instead of just catching on the third beat you are also going to simultaneously throw straight up from the opposite hand & horizontally from the same hand into the opposite hand as you would in a standard Box. Again make the vertical throw to about 6 inches above your head. As it peaks throw a swap & continue. Say to yourself, "Right, swap, Left, swap, Right, swap..."
Remember to always watch the vertical balls, there is no need to look down. With practise you can reduce the height of the vertical throws to shoulder level, the lower you go the faster your hands have to move.