TheThird your DS3 was being a selfish arse by the sounds of it so absolutely fine to tell him so. I certainly would have done and would have had no angst about dodgy parenting! Your poor DS1 - it's incredibly tough when a DS cries, especially a big DS, so poor you too.
Our main problem at exam time with eight DC born close together in a small house with three to a bedroom has been finding space to work. Somehow we're at the back end of it now, but I don't really know how they managed. That was proper bad parenting.
Anyhow, Edexcel maths here today and agree about the likelihood of a stinker. DD's main problem (even before the likely stinker of a paper) was our huge resident raven who decided to sit on her windowsill and squawk relentlessly from dawn. DD was seriously tired going off this morning but needs to make it through the day to do a bit of revision for English and history tomorrow, although just as well it's language not lit I suppose.