For acquisition and registration details of GPR data in zone ADMIA profile 18 and 23, we performed measurements from B-scan (2D) mode. The radargrams that have obtained (2D) can be combining to form 3D C-scan
You can combined and interpolate a set of 2D GPR sections to a 3D data volume and extract slices if you have access to for example ReflexW or GPR-SLICE software. You could also use Seismic Unix to do so.
You can use Reflex3DScan software, even demo version can combine 2D profiles to 3D, but after processing 2D profiles and unifying all lengths of 2D profiles