Ok let me jump in  

so each miner when solo mining will be trying to crack a block. it doesn't specifically try and crack a single block, it generates hash's at random, until it finds enough to generate one full block.
this means that each mining instance will be hashing away, it does not mean that they work on the same block, it means their both trying to figure out blocks.

lets give a simple example.

Rig 1 - 300khash/s
Rig 2 - 200khash/s

point them both at the same wallet, and that wallet has the equivalent of 500khash/s being sent at it. in theory every hash that is done adds together to try and make a block.
when mining you put in an ip address for the miner, these should both be the same on each miner.

you do NOT need 3 computers to mine, as the wallet can run on one of the mining systems.
in short, pointing 2 miners at 1 wallet adds up the chances of you finding a block.

similar to what DemosMirak said,
if you have a dice and you want to role a 6, and one computer roles 300 times (300khash/s) and the other roles 200 times (200khash/s) you role a total of 500 times (500khash/s) as they are working together (mining to the same wallet)

any more questions? i know its been dumbed down a lot but it should be easy to understand