Dax

Syntactically the first and second measure are identical. Min( sales[order date] ) == minx( sales, sales[order date]). The only difference is you have wrapped on in calculate, since there is no row context or calculate modifiers it does nothing.

The 3rd is wrong. You are summing the min order dates of each product together

It is not clear why your 1st and 2nd should have differing results 

The first two return a single date for the first order from any product. So you will only get sales from products on that had sales on that day.

When you iterate products you are getting the first sales date, and then the sales amount per product, then summing the results. So this also includes products that have first orders after the very first order date