Once you have the pattern down for two sheep, add a third sheep.
Throw the third sheep when the second sheep is at its peak
height. At this point, the first sheep should be passing
your hand. Don't worry about catching it at first. Just
get used to throwing all three sheep at the right times.
After some practice at this, you will want to try to catch the first two sheep. As you get the knack of throwing two sheep and catching the first two, try to throw the first sheep when the third is reaching the peak.
Finally, all you need is practice, practice, and more practice.
Good luck!