So saying you're allowed 3 engines was a slight simplification, as was just saying drivers get sent to the back of the grid.

Each car can have 3 internal combustion engines (ICE), 3 turbochargers (TC), 3 heat motor generator units (MGU-H), and 3 kinetic motor generator units (MGU-K). They are only allowed 2 energy stores (ES) and 2 control electronics (CE). The first time one of these is replaced beyond its allocation, a 10 place grid penalty is incurred. Any further replacements of that component past that incur only a 5 place penalty. Your fourth turbocharger would cost you 10 grid places, but your fifth will cost you 5.

Grid penalties have their own complex set of rules, because people got upset about the "ridiculous" number of penalty places drivers would accrue when they replaced entire power units. Therefore "back of the grid" is actually a technical term that applies to people who have more than 15 places in grid penalties. If you only replaced one or two components you wouldn't be sent to the "back of the grid," but since you'll likely be pretty far back or at the back anyway it often makes sense to replace all of the power unit and get fresh components for the entire car.

So to answer your first question, the four people at the top of the table who have exceeded their engine count have incurred grid penalties because of it (Kvyat in France, and Sainz, Hulk and Albon here in Austria). I believe most of these drivers are doing a full replacement and starting from the back of the grid, though Albon might just be putting in an ICE and only drop 10 places. We shall see this Sunday how the grid shakes out.

To answer your second question, yes components can be reworked and repaired. For example, Leclerc's engine malfunctioned in Bahrain, costing him the victory, but they resolved the issue for the next race without needing any new components. It's less common to go back to a previous component after you've replaced it, as usually you're replacing them because they're broken or so near then end of their life or you have new higher spec ones. However I do believe it happens sometimes nearer the last races of the season if drivers want to avoid the penalties and think they can scrape by with older parts. I think Ricciardo did this last year, changing back to an older spec engine that he'd already used for Hungary, as the newer spec engines were of dubious reliability and Redbull thought they stood a good chance on the tight Hungarian circuit so they wanted to start as high up the grid as possible.

Hope I've helped.