It's just another G chord, only with a different selection of notes. A G chord is anything containing only the notes G, B and D. 3rd fret on the B string is D.

This is assuming you don't play the top E string open, or that you put another finger back on 3rd fret on the top E string. If you do play the top E string open, that's G6.

The normal way of writing chords is to give a list of 6 numbers, where the left-most number is the fret held down on the bottom E and the right-most number is the fret held down on the top E. Open strings are shown as 0. So a basic G would be 320003, and your G6 variant would be 320030. I guess you tried to do ASCII-art for the chord diagram, but unfortunately it doesn't work in Mudcat - you need to know some HTML to format it correctly. :-( Using numbers as shown avoids the problem.

Graham.