Long Answer: 3"/1m accuracy cannot be achieved reliably at present by dedicated WAAS enabled Aviation GPS/VHF devices further enhanced with LAAS [Local Area Augmentation System] using differentially corrected signals via VHF from GPS Ground Stations around an airport, although it's expected that further refinement will result in consistent accuracy of or below 1 metre for all-weather precision approaches. Therefore it would seem unlikely that there is any 'portable' GPS/GLONASS/WAAS device available which is capable of 1 to 3" accuracy without the type of positional correction available to LAAS enabled equipment.
Short Answer: Dunno.