SB+SoC onto one of the mobs, then SB+SS onto that same mob, then spam SoC on that same mob. Continue until the group is dead, short burst damage is what we do best
. DO NOT worry about your dps on trash, theres a reason why it's called trash. If someone in that group says something about your dps on trash, point and laugh at them. This happened while I was in Scarlet Halls not to long ago. I had come in late a little bit before the last boss. A demo lock was talking a big game and while on trash was still talking a big game, he even mentioned something about my dps on trash. I simply stated, "Uh....this is trash bro....also you are demo rofl" more big talk from him. Last boss of Scarlet Halls comes up and normally I don't use doomguard in dungeons but this is a rare occasion to silence a baddie. Before we pull I say "You want deeps bro?" in caps of course and at that moment the shaman in group blows bloodlust and I blow all cd's naturally. I reach a sustained 216k dps on the boss, type out "Look how good I push buttons" and leave. I think he managed to crack 110k on the pull.
Now, any spec will work in any situation. Affliction is perfectly fine for 5 mans, follow the guides on this site for trash just simply do what I mentioned above for 5+ mobs. For groups of 3 just dot up each mob, if the mob dies before you can get dots on it oh well....it's trash. However, if your group is dying on trash and cannot move on through which is almost impossible to do I would probably switch out for demo as it is the highest theoretical aoe dps spec. Again, if someone is complaining about your dps on trash you can immediately write them off as either a troll or a baddie.
In a raid setting for trash as affliction:
SB+SS onto a mob. Put a haunt onto that mob and use SB+SoC onto the same mob. Then spam SoC onto the dotted mob, this is affliction aoe.
The ideal way to aoe 5+ mobs as affliction would be the same as above but before you start to spam seed of corruption you throw out CoA onto all of the other targets affected by your corruption and then retarget your fully dotted mob to start spamming SoC.
For at most 3 mobs, all you want to do is have CoE on all 3 plus all dots and keep haunt up on your focused target, that's it.