|I don't use Google Docs, but according to this 2011 thread, Google Docs does not allow 3D references. As far as I can tell, you will need to create a SUM formula referencing each individual sheet either before you upload the file or once it's in Google Docs.|
In addition, while researching this, I found a couple of other threads that mention people having problems with even simple formulas changing when Excel workbooks are uploaded to Google Docs. Just be aware.
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