The only way you can run the three barb H/C is having supplying water from the H/C going to the pump and from pump to water block. This may or may not be the best setup. You always want the coldest water entering the block first and you can not do this with a three barb H/C. Maybe somebody has some insight as how much hotter would the water be after going through the pump first. I do agree it would look better than using a Y fitting.
Photo of the heater core already made.
