I wonder, if it´s possible to have a Stdev function in forms(in detail part)

I have 3 unbound fields in the form and i want calculate the Stdev of these 3
fields. Is it possible i detail section?
The three fields and the Stdev result should be on the same post. I have
tried this: =Stdev([field1]+[field2]+[field3]) and also several other
solutions but it does´nt work. Is it possible?

Grateful newbie

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