Platinum is installed on my DAW at home but X3 is what's on my Church laptop, which is what I use to connect to the instruments there.  If I save a file in Platinum, will I be able to open it in X3?

Yes, you should be able to do that. However, if you used any new features in Platinum, those will not be editable in X3. Also, if you have any different plug-ins that you used on your home DAW, those will also not be accessible.
 
Scott

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Scott R. Garrigus - http://www.garrigus.com
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So far, the only exceptions are projects using patch points and aux tracks. These projects cannot be opened by versions lacking the feature.

Thanks to both garrigus and scook!