I am a newbie to all this so am not too tech savvy. I have a disklavier piano and want to convert the midi recordings into audio. I have done this successfully using a conversion program and SF2 soundfonts.
However, after coming across the Grand 3 virtual piano suite I really want to use that. Is there a simple program or process to make this work. It doesnt need to be complicated - no editing etc required, just a straight conversion… Any advice is welcome. Thanks!
You didn’t list which version of Cubase you have so I’ll just give a generic answer…
Export your midi track to audio by using the “export” function under the "File menu. There (depending on which CB version) you can choose what type of audio file you want. Instructions for this sn in the CB8 op manual starting on page 938.
Regards 
It doesn’t seem like the OP has any version of Cubase, he’s looking for some software to get the job done.
Any version of Cubase will do but Cubase isn’t free and although it’s certainly great for getting your midi to audio, it can do way much more than that so it’ll be daunting at first, and if that’s all you use it for it’ll be too expensive to justify.
What you need is a VST host that can accept incoming midi (If I understand you correctly, the midi is on your piano and you play it back? Or do you have midi files on your computer?), host the Grand and record the resulting audio.
I have no experience with free VST hosts but check http://www.kvraudio.com, I’m sure they have listed a couple.