You are mixing things up here
First of all, assignment of values in JS is done with a single =, not with 2. So, your NumericField3 == "0" is a logical expression, not an assignment.
Second, RenoCost and NumericField3 are objects. You must ask for the value of the object using object.rawValue.
So, what I would do is this:
In the calculate event of NumericField3
var reno = (0 + RenoCost.rawValue) // rawValue returns a string
var calcValue = 0
if ( reno > 50000 && reno < 100000 ) {
calcValue = reno * 23 / 100 ;
}
// this last line tells the event what to put in the field, it looks a bit weird, but that is the way to do it in the calculate event, the last line is just a value/variable
calcValue