3 balls in 1 hand, a.k.a. 3-in-1, is definitely not an easy pattern, mostly because of the speed, accuracy and strength/stamina required to do it. I could learn to do it because I came up with good prerequisites, which I will describe in this article. It still took me a few months to learn it, though, and I still haven't mastered it.
Throughout this tutorial, I'll assume that you want to juggle 3 balls in your right hand. If you want to juggle with your left hand, just mirror-invert all instructions here. In all siteswaps mentioned here, the first throw will be from the right hand.
First of all, you should have mastered the 3 ball shower (siteswap 51) to almost perfection.
There are 2 mirror-images of the shower.
You must perfect the one where the left hand makes the horizontal pass
(i.e. siteswap 51, not 15).
Then you must learn to make a type-6 throw.
While juggling 51, replace one of the type-5 throws by a type-6 throw.
It'll look similar to 6151505151515151.
Practice this until you find it easy to replace a type-5 throw by a type-6 throw.
Now practice 615150, then 6051 and then 606150.
You may also want to try out other variations. After you get good at these, you can finally try 3-in-1.
You may also like this video tutorial by Chris Taibbi.