Help to configure

Hi, i tried to configure 3 fields.

The first field i have a select filed with 3 option with values e.g 1 2 and 3.
The field 2 and field 3 i have a input type field.
The last i have the result with a simple formula

field_1 + field_2 + field_3

And i write in result section {result}
But did not worked!
Some tip? Help me please.
Thanks,
Rafael

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