Most SIP carriers have per call duration limits, generally to help reduce toll fraud (which is still a very real thing)... In my company on our hosted platform we use 7 hours, and I have heard other companies use 4~12 hours, although 3 seems kind of short but I think this could very well be a carrier thing.
One additional possibility, although at 3 hours it makes the possibility pretty low, is that your firewall is dropping the call due to long a session being open.