it is possible that the return side of the heating circuit joins the main return BEFORE the hot water return, meaning that some of the system will heat up back to front when the 3 port valve is positioned for hot water.
if you have limited underfloor access then use two push fit reducers and come through floor boards in 10mm.this would harldy affect performance and would be easier to work with than even the flexible copper you mentioned.you would also need two reducer sets to go in the valves.
