No Soldier shall, in time of peace be quartered in any house, without the consent of the Owner, nor in time of war, but in a manner to be prescribed by law. (source)
This is a reactionary Amendment. Basically, during the time right before the Revolution (and during, I’m sure), the British tended to tell people that they needed to share their homes with their troops during war time. When the war ended, the British decided to keep on quartering soldiers in private homes during peace. Its one of the items that started warming up the revolution. It’s hard to imagine that happening today, but the men who wrote this document wanted to make sure it didn’t.
There isn’t much to say on this one. As far as I know it’s pretty much never debated that the Government could actually come to your home and say “Guess what, Roomies!?”
For a more modern example, I googled the third amendment and found a case in Nevada where a homeowner claimed the local police violated their third amendment rights by forcibly invading their home to use it against a neighbor they were investigating and staying for 9 hours.
It has also started to come up to relate to surveillance state by police/government but its debatable on whether the amendment would include “cyber soldiers.”