I would say it's pretty safe if you know your way around G, great as something to chill yourself out, or alternatively get rowdy or if you can dose properly it could serve to gain some sleep after a long trip if you are still feeling wired.
Definitely done K after G [to reduce the libido ] but never tried all 3, acid always potentiates ketamine for me so if such a deadly combo was to be used do the K slowly, one bump at a time