Add Selected Numbers From Across Fields And Display Real Time

I am trying to display a total based on 3 separate combo boxes in a form. there is a name with an associated number value using 2 fields. the values come from a linked value spreadsheet. the name field is "text" and the number is "number". the user selects their name and number using a combo box drop down window. this occurs 3 times in my form and i want to sum them in a 4th combo box. this is about how it should look, where "Total" happens automatically depending on the name selected.

Lewis 3 Smith 2 Parks 2 Total= 7

thanks for helping.

dave