GA 3 is not supported anymore by Steinberg. They are all in on GA 4 (which is quite another product, and in my opinion not as good as GA 2&3). You have not completed your profile so it is a little difficult to answer your question (please do for further posts). But I think GA3 stopped working on MAC on Lion. If you are on Snow Leopard (or earlier), GA 3 should work, but on later OS it won’t. I may be mistaken on the OS but I am on Mountain Lion and GA 3 does not work here. Nor does a bunch of other software. I am running older software on a PC and link the two computers together. Thus I can access GA 3 on my MAC. In order to link them you need either a sound card in both computers and a digital connection between them or an ethernet connection and the product VePro installed on both computers. Both solutions are quite expensive. You also need a dongle with the GA3 license on the second computer and for the digital sound card/sound card solution you also need a Cubase license on the second computer. I would not recommend this if GA 3 is the only software. I am not sure if GA 3 will work through VePro but I guess Vienna (developer) can tell. Then I would rather suggest that you have a look at Toontracks EzDrummer. Think they still have a light version that is free or at least a demo you can try. Also note that you have a light version of GA 4 (G4SE) included in your Cubase 7.5.