The Switch3 operation in the Play and Play GTX is purely to mimic the Loop functions. It essentially breaks the looper controls out, so that you can still have full access to preset up/down, hit, tap tempo, key/scale (if it's set to UP/DN in setup) while having the 3 looper buttons available at all times.
is there any reason to hope for more functionality of switch3? i am thinking about independent guitar sound control or possibility to assign a patch directly do buttons 1,2,3? Or maybe are you planning some midi controller support? I am ordering gtx today but as it is now there is no reason for me to order it with switch3.