You are of course right about frail sweepers - but that's why people use Garchomp/ Tyranitar/ suicune/ Salamence/ Lati@s. For me, sash and sturdy aren't enough to warrant SR. You won't always get up SR, and when you do the majority of the time it won't make a large difference. The subway is all about majorities - being good against most of opponents. There are better ways of setting up sweepers than SR in 3v3.Yes, the other two Pokemon in the team must be good for it to work, but even if you had three fast, frail sweepers, you would run into a problem which a SR-lead alleviates (Spelling fail I think): Focus Sash and Sturdy Pokemon. Frail sweepers would get a hard hit in, hit the opponent down to their Sash/Sturdy, then the opponent would either get a lucky hit or kill you outright due to your frailty. With SR up, though, your sweepers can, well, sweep without worrying about Sashes/Sturdy-abusers.
No, you're still going to get one-turn-revenged with or without SR. SR is doing about 6% damage to 2 pokemon. That's about 12% damage average per battle, that's not worth it. You're losing a slot that you cannot afford to lose - no two pokemon even come close to being able to cover everything. Avoid simple 1v1 countering thinking as well. Say you have skarm/ phys sweeper/ special sweeper, and you go against an opponent with fast, special attacks. Chances are, you'll lose skarm. Then it's 2v3 where the 3 are at basically 94%ish. Unless you outspeed and OHKO everything (a rare occasion), the battle is going to be very close. Try using just two sweepers and you'll see what i mean. Garchomp/ starmie loses to far too much. There's a lot that outspeeds both of them, starmie is suprisingly frail, they fall short of complete coverage - Peterko couldn't get past 49 with Garchomp/ Starmie/ Skarmory, and he's experienced (look at the platinum records).Basically, you're using a slot on your team to protect your teammates from being "one-turn-revenged" (A term I made up to describe when an opponent lives due to Sash/Sturdy, and proceeds to kill your Pokemon in the same turn) so they can successfully sweep through the other person's team. Also, you're probably gonna want to use a Special sweeper and a Physical sweeper, like the person that thought of using Skarmory suggested Garchomp and I will too, and I will also suggest Specs/Life Orb Starmie as your Special sweeper, which resists Ice for Chomp (even though neither of them can take a hit, a resistance is nice) and Fire for Skarmory, while Chomp resists Fire (for Skarm) and has an immunity to Electric (for Starmie and Skarm)
A lot of strategies look or seem very good, then you lose a few times. I really recommend just playing a bit and going from there because with all due respect, it seems as if you don't really understand what these 3v3 battles are like. You want most wins to be 3-0, 2-0 should be a "close call". You can't afford to continually put yourself in situations where one crit, one hydro pump miss, one flinch, loses you the match. It's fundamentally different than 6v6 against another player. The AI just wants to knock out each pokemon in succession, it will not consider the match as a 3v3 but 3 lots of 1v1s (like bad players do), it's exploiting this programming that ultimately leads to very good strategies - such as trick scarf.