automatically assign a contract to sc

Hi Experts

We are working in Classical scenario.

While creating a sc, when we click on the source of supply, we get a list 3 contracts.

All the contracts are proposed and none of them is automatically assigned.

Our requirement is that one of the these contracts should automatically get assigned as the source of supply, and when we delete this contract then we should get the list of 3 contracts to choose from.

thanks

gaurav