Someone has a hidden smartphone and are using the internet in a 3G connection.
- find where is he? Maybe using a signal detector.
- find who is he? Maybe know the MAC number.
- just find if someone is using 3G in that place?
There are hardware devices available that will detect a 3G signal. Pulling the MAC address or such involves intercepting the traffic, which has legal implications, so consult a lawyer, and be prepared for them to yell at you for even thinking of intercepting phone calls without a warrant.
There are also devices that can jam a 3G signal. Again, there may be legal implications.
Lastly, as so often is the case in InfoSec, be careful not to get too hung up on technological solutions. This is also a human issue. Have you made it clear, in your policy, your employment contracts, and in your employee communications that using a 3G device will result in immediate termination?