Ground wire

I using the iotavx sa3/pa3 paired togther.  The sa3 has one ground thumb screw, located on the back panel. How much gear can I put on that one ground screw if need be? I will be getting a turntable and 2 rel t5i subs soon and although I don't know yet if I will need to use the ground wire on the subs for high connection... I really don't know if it's cool to put three or more on the ground screw of the sa3.  If it's ok... is there a way to do it? If I need to ground the two subs... would I just twist the two togther? If I connect the turntable.. can it touch the ground wires from other gear on that screw? 
Thank you.  :)