A new type of RPC chamber prototype, consisting of a triplet of 50×100 cm2 RPCs, having 1 mm gas gap, 1.2 mm electrodes and new high sensitivity front end electronics, has been designed for the HL-LHC ATLAS upgrade program. Beam test of this prototype chamber was performed in GIF++ using 100 GeV muons and a 14 TBq 137Cs gamma source to simulate the HL-LHC environment. The amplified analog signals of the chamber have been read out by 32 channels of high speed digitizer, permitting to study in details the various aspects of the detector physics in different condition of gamma background and field applied in the gas. Analysis methods and results of these data will be presented, illustrating in details the most relevant features of this new detector: ∼98% efficiency, 400 ps ∼ 500 ps time resolution and ∼0.1 cm spatial resolution, cross talk in between the singlets and cluster size.