From MagicPedia

Two in the Hand and One In The Pocket a classic magic effect done with any small object (sponge balls, sugar cubes, balls, coins or marbles). Two are placed into your hand and a third one goes in your pocket. Magically, the third ball travels from your pocket back to the others in your hand. This is repeated and just when the audience thinks they are catching on and believe you have three balls, all of them vanish.

It was called "The Gadabout Coins" in Modern Coin Magic.

References

• The Three Paper Balls in 100 Tricks You Can Do by Howard Thurston (1912)