Well really I just keep the ranking order as in each source site and intercalate them, like one HN one Reddit, one Slashdot, all keeping the order of each front page.
I'm still thinking on what would be the optimal way to display them, but didn't heard of http://serializer.io is it basically the same concept? the only site I found doing a similar thing was http://reader.one/ but its IOS app is not so nice.