How can I determine a duration between a starting date and ending date or today's date only calculating after 3yrs from the starting date?

Not tested, but try this as a Formula Duration field.

var date StartPlusThree = AdjustYear([Starting Date],3);

var date EndDate = if(not isnull(Ending Date]), [Ending Date]), Today());

Max(Days(0), $EndDate - $StartPlusThree)