|A 3MB file size really shouldn't be a problem, at least not as the direct cause.|
A couple of things to keep in mind and to try:
When Excel opens a file, it puts a temp copy of the same file (same size) in the same directory. It doesn't delete this temp file until a save of the original is complete. Therefore, in order to save the file, you have to have twice as much disk space as the actual file size.
I assume you've checked the available disk space and it's well abouve the 6 MB required, right?
Have you tried saving it locally to see if the problem still occurs? That would confirm or eliminate the network as the issue.
Google the problem - it appears to be a fairly common issue, but not one with a single, concrete solution. maybe something you'll read will point you towards a solution.
Try this article for other suggestions.
Finally, as a work around, you might try some VBA to save it locally (assuming that you can) and then copying the file to the network drive. I'm pretty sure this could be done with a Before_Save macro stored inside the workbook. That would make it fairly transparent to the user.