Handling Multiple calls using GSM

Hi, I have a requirement in Asterisk, please help. Lets say I have 3 GSM SIM cards, each connected to Asterisk using an FCT which inturn is connected to an FXO. What I want to implement is, I want to expose the number of only 1 of these sims to the outside world. But if a call lands on that SIM, depending on whichever is free, I want 1 of the 3 SIMS to take the call. The calls between these SIMS is free, so is there any way I can do this?