It's really difficult to play traditional baseball with three people, especially when two of those people are under the age of six. Our family, however, is unsurpassed in the art of adapting. For example, we spent our afternoon in my parent's backyard playing Not-Quite-Baseball.
The casual onlooker would no doubt recognize our equipment...a traditional child's bat and ball. The player's rolls, on the other hand, may seem more foreign. In our game, there's a pitcher, a batter and a finish sayer. While you can probably guess what the first two do, I'll explain the third. The finish sayer imparts words of wisdom on the other two players after every hit and before every pitch. These can be suggestions or rules, such as: "Don't throw the ball too high or my puppy will sting you." and "Don't cut down trees, only hit the ball."
After a ball is hit, the roll of the pitcher is much more hands-on. The pitcher is responsible for retreiving the ball and chasing the batter around the bases. If a pitch doesn't result in a hit, then the *batter* picks up the ball and chases the *pitcher* with it! The running bases thing has also been modified. With only one batter, and a tremendous chance that each hit only gains one base, we have to have the batter bring the bat with them and hit from the base that they're on at any given time. This makes it extremely convenient to have the pitcher's mound in the middle.
You should try playing this for a while. You'll never look at baseball the same way again!