This is how to learn to juggle three balls.

Start with one ball in your less dexterous hand. Hold the ball out in front of you with your elbow bent confortably at about 90 degrees.

Throw the ball to you other hand such that the ball reaches apogee at about eye-level. Catch the ball and throw it back to the first hand.

Do this a few times, back and forth, trying to keep your hands in roughly the same position. This sounds stupid, but just do it. Do you know how to juggle? Do you want to know how to juggle? Then do it.

Next start with a ball in each hand. With your less dexterous hand, throw a ball as before. When the ball reaches eye-level, throw the ball in your more dexterous hand. Try to catch both balls.

Do this for a while.

Now start with two balls in your less dexterous hand and one in the other. Throw two balls, just as before, then when the second ball you threw is at eye-level, throw the third ball. Don't panic and for goodness sake DON'T TRY TO CATCH THEM, just let them fall. They should hit the ground in a pleasing, even pattern:

plop - plop - plop
as opposed to
plopplop ... ... plop
or some other variation.

Do this until you get bored.

Now start trying to catch them. This is the tricky part just try to catch all three balls, don't re-throw them. When you can catch all three without running around too much, start re-throwing the balls as you catch them.

Juggling is just one of those things that your muscles and nerves learn, like riding a bike. This means that you will practice and not be able to juggle, and practice some more and not be able to juggle, then you will wake up one morning and suddenly you will be able to juggle.