In theory this is how to do it:
Physical Interfaces > LINE Port > InboundCallRoute:
{ph,pp(ob290123456),pp(ob290123457),pp(ob290123458)}
However, I do not know if OBiTALK will accept three calls at the same time. In any event you would have to change:
Voice Services > OBiTALK Service > Calling Features > MaxSessions from its default 2 to 4. In theory this will then allow four simultaneous calls.
The maximum number of devices allowed using "call forking" is 4 and "ph" counts as one, so that's your limit.
If the above does not work, you could "daisy chain" the devices if the second one is an OBi device, not an OBiON/OBiAPP. Get the first to forward to the second and the second to forward to the third.
Let us know how you get on. It will be interesting to see if OBiTALK allows 3 simultaneous calls. I know it allows for two like most voip providers.