I have a current 3-lock team that's at 52 right now (see my blog in my signature). I also have an S3 lock that's Affliction to Unstable Affliction with the rest of the points in Demo. I've also raised 2 other locks to 70 on other servers... so I know just a little about locks. Let me tell you my experience...

Levels 1-49:
I leveled from 1-49 as affliction, getting Siphon Life, insta-Corruption, Fel Concentration, etc. That a lot of fun, with little to no downtime. Usually I roll with 1 void, 1 imp and a succy/felhunter. Blood Pact is really nice and you only need 1 blueberry to hold aggro. I had a /castsequence macro to cast Siphon Life, Curse of Agony, Corruption, Immolate (in that order... normally it's longest DOT first, but the life gain from Siphon Life outweighs that). I rarely fight more than 1 more at a time because it's too much management and quite a bit of downtime afterwards to drink up but I have been able to easily take on 6 mobs 3 levels above my team.

Normal fight is all 3 locks have pet attack at the same time as 3x Siphon Life goes on. Then it's 3x Curse of Agony. Then wand... don't neglect your wand! 3x wanding with a high DPS wand is a lot of damage. Mobs go down really really quick... we're talking less than 5 seconds here. Sometimes I Drain Life after the set of dots... I rarely need to go through the entire cast sequence. I Drain Soul as needed for shards. One thing about draining is that you have to make sure they auto-follow your main after the fight since draining will break follow.

Other pulls include sending in the pets all at once while initiating 3x Shadow Bolt. Then wand again.

These methods work really well even though I'm going against the traditional Aff-lock fight of pulling multiple mobs and letting the Dots do their thing. I find that I can go through all the mobs in an area extremely fast even though I'm going one mob at a time... with no down time.

Level 50+:

I re-spec them ALL to Felguards. I was worried about the loss of insta-Corruption more than the loss of Siphon Life. But man was I wrong. Now the cast sequence changed to Curse of Agony, Corruption, Immolate. The fight has 3 felguards attacking a mob with me simultaneously casting 3x Curse of Agony. Then wand. Mobs even level or higher are dying even faster... 3 seconds max. The Felguards are doing extremely high damage. I turned off auto-cast on their Cleaves to save mana... just have Anguish up. And they can hold aggro very well.

Last night I took on 8 mobs at once in Shadow Hold in Felwood that are 3-4 levels above me. I lost 2 Felguards, but was able to kill them all ending with about 1/2 life and almost no mana. Now that I'm 52, my locks are gaining back insta-Corruption slowly even though I really don't need it.

Trust me... go Felguard as soon as you hit level 50 on a trio. You'll mow things down really really fast. Conventional wisdom say that you should wait until mid to late 50's to spec Felguard with a single lock if that's how you want to go, but with a trio, go Felguard as soon as you can. I guarantee you will not regret it.