Easy: put three rows of 10 bricks (each representing 0-9) below the brick that fires a relay. Have the relay fire the projectile three times. Each time a brick is hit it would disappear. Thus, if the projectile is fired in the same position three times you'll have an outcome of three equal numbers on the slot machine. Now to actually display the numbers with prints... I don't know at this exact moment.
That has been done before and works as a concept perfectly fine :p
We did it at the Blockland Custom Cup thing at the end of summer.
Creating a slot machine with default events is pretty easy when you know little neat machine things like in the OP. Just use some logic (like the above quote from Lego), cancelling events and timed delays; Hey presto you got a slot machine :D
