Your enums have a caption and a value combination like 1 1, 2 _2......10 _10. In the microflow you create a integer variable for every enum and give it the value:\ parseInteger(getCaption($Entity/EnumAttribute1)), parseInteger(getCaption($Entity/EnumAttribute2)), parseInteger(getCaption($Entity/EnumAttribute3)). In the third action change the TotalEnum field by adding the three variables in a change object activity with a refresh to show the result on the screen.
The total enum field should in this case be a integer as I'm adding integers, but probably using parseFloat will allow you to have the addition in a total field of type float.
I hope this helps.