As far as I understand, the Garnet VM emulates Palm OS 5.x, so your program that requires Palm OS 3.0 minimum should have no problem.
If you wish, in the configuration dialogue, you can choose a device model for each installed Palm app--you have your choice of:
1. Treo 650
2. Tungsten T3
3. Tungsten T5
4. Palm T|X
5. Custom

For the Custom choice, you get to input a Company ID and Device ID. Here is a link to a table where I think you can find the right codes.

http://homepage.mac.com/alvinmok/palm/codenames.html

As far as I can tell, you only need to specify the Device Model IF your application has trouble launching. Otherwise, choosing "default" for the Device Model should be fine. All of the other choices (other than Custom) are Palm OS 5.x devices.

As an aside: Changing the Device Model with the configuration dialogue will NOT change the Device ID that your computer sees during a Hotsync. There are other posts on this forum for manually changing that information in the gvm.store using Terminal.