Most efficient way to rush Act 3?

What is the fastest chain of instances for running Act 3 without relying on WPs from other people?

Longest act, and also the most boring :/

Is it worth doing the sewer quest during leveling, or faceroll it afterwards?

you get two of the busts by traveling through the sewers so unless you decide to not pick them out I always grab the third one of I trip over the secret grate in the market. it is usually (always) around the wp.

I always take a Docks leveling party for get that wp, from where is easy to move quickly.

If not that, just set yourself up with quicksilver flasks up the butt and run yourself to wherever you need to get to. Usually act 3 I will do the busts quests for the reward, and then run my ass over to docks.

It may be long and boring, but it's also pretty linear. You don't really have much choice in what order to do the zones in unless you're getting waypoints from someone else. I would definitely do the sewers. The skill point is always useful for something. Go to docks before temple so you don't have to make a second trip back there. The Fairgraves quest can probably be skipped altogether. Library quest might be worth skipping on Normal and Cruel for some builds if you don't need those gems, but merciless is almost always useful.

You asked about chain of instances, so I will give you exactly that.

Assuming you want the skill point (because this is not some 2-hour race to kill Dominus, it's a fairly long-term league where skill points matter more) and are doing this solo (no free WPs):

Slums->Crematorium->TP to Sarn Encampment->Slums (or if the sewer grate was very near the crematorium entrance, WP to Crematorium then go back to Slums)->Slums Sewers->Warehouse Sewers (tag WP)->Warehouse->Market (tag WP)->Market Sewers (TP out and use wp to go next)->Market->Battlefront (tag wp if you find it while rushing to docks, but definitely pick up ribbon spool because I think it's required before Dialla makes you the Infernal Talc)->Docks (get sulphite, then get WP if and only if you already found battlefront WP in which case you use it)->Battlefront->Solaris 1->Solaris 2->Warehouse Sewers->Sewer Waterway->Ebony Barracks (tag WP)->Lunaris 1->Lunaris 2->Lunaris 3->Ebony Barracks->Imperial Gardens->Sceptre->Upper Sceptre

Edit: there are more efficiencies possible if you want to position Portals intelligently and use "go to character selection" to get to town for convenient use of them. But it's unclear if you want to be actual-racer crazy.