That is too easy! You create 3 SIP accounts:
123@sipsorcery.com,
124@sipsorcery.com and
125@sipsorcery.com .
You create your 3 SIP Providers. Tick the Register box on each provider.
For provider1, enter the Register Contact as
123@sipsorcery.com
For provider2, enter the Register Contact as
124@sipsorcery.com
For provider3, enter the Register Contact as
125@sipsorcery.com
That's all that's needed. No need to enter anything in the In Dial Plan.