Take a look at this for a solution
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I use a field info tool to get the field names.
Then a sample to get the last three field names.
The recordId adds a new field which I will use to rename the last three fields using the Dynamic Rename tool.
Now my last three fields have names I know ([1], [2] and [3]) I can write the formula I want.
Then use a final Dynamic Rename tool to put the field names back to what they were originally.
Hope that does what you wanted.
You can download the module here
http://downloads.chaosreignswithin.com/community/SumOfLast3FieldsDiv3.yxmdAdam
Adam Riley
https://www.linkedin.com/in/adriley/