I would say thats possibly a phone fault. If its losing signal and everyone else around you on 3 isnt complaining what are the chances its 3? Have you been into the network settings?
If not go to Settings, Networks (or More Networks) then Mobile Networks. Mobile Data needs to be on, Network Mode keep to LTE/WCDMA/GSM(auto connect) or WCDMA/GSM(auto connect) as any other option will result in either picking up 3G/H+ or E/G only.
The phone should automatically pick up the 3 mobile settings and that can be accessed through Access Point Names or APNs. It should show one or two settings and one will have a green circle to show its selected. After a factory reset this may need to be selected again to work as it goes back to basics. If those are all checked its worth getting the phone checked out.