How to implement GetHashCode() in a C# struct
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Always the latter. The former isn't sufficient because most bits are 0 (your numbers are most likely small), and those zeroes are in the most significant bits. You'd be wasting a lot of the hash code, thus getting a lot more collisions.
Another common way of doing it is to multiply each item by a prime number and relying on overflows:
return unchecked(FolderID.GetHashCode() * 23 * 23
+ SubItemKind.GetHashCode() * 23
+ SubItemID.GetHashCode());
Edit: Updated to use unchecked for explicit overflow support as per stakx's comment.