PGWo, DS and DD is MumsNet shorthand for 'Dear (Darling) Son' and 'Dear/Darling Daughter'. So it follows, DM for example, would be Dear Mother, etc.
"For anyone who knows, how is the lifestyle different for the scholars and commoners in these three schools?"
I can only speak for one school - Eton - as my son was a KS (King's Scholar) there. I suppose there are a lot of similarities between Eton and Winchester in particular because Eton was founded in 1440 by Henry VI along the image of Winchester which was founded some 48 years earlier. So we are talking about medieval times here and I start from here because you said in an earlier post that you didn't quite know the history.
In fact, Henry VI "persuaded" a lot of masters from Winchester to come to Eton in its infancy and even roped in the best students, some 25 of them first but later 70, as the first students to start at Eton. His idea was to educate 70 scholars free of charge; all to be fed, housed and educated at his expense.
A slight deviation here: You mentioned your son (DS) has great designs to go to Cambridge. That being so, ETON is, and should be the natural and default choice out of the three schools mentioned here.
To continue with the Eton story, Henry VI went on a year later in 1441 to found the majestic King's College at Cambridge. This is one of the most impressive Colleges of all Cambridge University colleges! Following their (basic) education at Eton, the scholars were to walk into King's College Cambridge to further their education after which they were to become the King's advisors in all matters of state.
But after almost 600 years many things have changed. Unsurprisingly, these medieval schools still retain many of their past traditions but mostly of a ceremonial nature. At Eton there are still 70 KS's at any one time and they are distinguished from the rest of the 95% Oppidans (called Commoners at WinColl) of the school by wearing a gown over their school uniform.
The KS's live centrally at a house called, College right smack at the heart of Eton where everyone has his own private bedroom - as does each and every Oppidan who lives in the other 24 houses spread throughout the entire village of Eton; something no other boarding school in the !and can match!
KS's dine exclusively at the prestigious College Hall (not to be confused with School Hall) where past guests and diners included the likes of Elizabeth I, George Orwell, John Maynard Keynes, Harold Macmillan, to name but a few. The food served at College Hall will seriously embarrass the restaurants of 5-star hotels, believe me. My other half just couldn't wait to dine at College Hall each time we visited for official school events.
On a daily basis, there is absolutely not much difference between a scholar and an oppidan. The boys are streamed according to subject and academic ability. It is therefore, not surprising sometimes to see scholars of the same cohort and ability spread over three or four different classes mixed with oppidan. This is not to suggest the standard of scholars has fallen, rather the standard of some oppidans is as good as, if not better than, some scholars. The environment everywhere is super-competitive. The atmosphere is such that you don't know where the next competition is coming from, College or Oppidan Houses. Not ashamed to say DS had been caught many a time "with his pants down", so to speak, when he was pipped at the post for the top prize from unexpected corners. No hard feelings - you win some, you lose some! It encourages you to work even harder if you lose. DS had his fair shares in winning. So please do not assume Eton is not academic.