I would like to be able to have the pages lay out in a 3 by 3 grid (or more) with crop marks on the paper when printed.
I can tell you how to do that part. If you have a frame to represent the size of your card (maybe it's the text frame that holds your data merge fields), you can select it and go to Window>Utilities>Scripts and select Cropmarks.jsx. Double-click and it will give you a window where you can set the length, offset position and stroke weight of your crop marks. When the marks are generated, you can run the data merge and each instance of the merge will contain the marks (really, anything on the page will be included). If you select Multiple Records from the Records per Document Page section and check the Preview checkbox, you will see how many of your item will fit on the page. You can then switch to the Multiple Record Layout tab of the window and see how changing the available settings will affect how your merge will look. I have read that before you click the OK button to create the merged document, it might be a good idea to uncheck the Preview box.
Also, to complicate matters, some of the cards have extra long text I would like to auto-flow into a second (B-master) or third (C-master) page only when necessary.
Sorry, I can't help you with that. I'm not sure it can be done with data merge, but if it can, someone here will likely be able to tell you.
InDesign's built-in Data Merge is plain vanilla and I don't think there's any satisfactory way you are going to get your extra long text to do anything but go overset using it. How does flowing the text onto a second page help with a playing card?
Thanks! I missed that part of the data merge help document. I guess the only problem then is the part that says: "You cannot merge multiple records if the data fields appear on a document page in a document with multiple pages, or if data fields appear on multiple master pages." So if I do multiple page auto-flow it's not going to work so well for me.
Nice pre-defined script too, I can probably use/modify that for my purposes.
These are more like system reference cards that hold rules text etc. as a useful resource for the player so they don't have to go digging through the books to find the text they need in each situation.
There is an auto-flow option that will automatically create new pages, but I'm not sure if it's going to work with the merge multiple records to a page feature. I might have to dig into scripts to do what I need.