As long as you keep damaging the other mobs and cycling flash you should be able to hold 3 mobs easily, the more that get added the trickier it gets. If you have shield oath activated then its fairly easy. Ultimately this is all dependent on whether or not your DD's can sleep the adds or if they're stupid and attack the wrong ones all the time. Those 2 factors make tanking adds easier or harder.

Flash adds enmity but not as much as a RoH combo, so its good for capturing the adds before you start dmging them.

But I usually label first and foremost by the potency of the target.

Shield Lob
Flash
Start RoH combo on mob #1
Start RoH combo and use RoH on mob #2
Flash
Start RoH combo on Mob #1, hit each mob #2 with SB, RoH on 3rd.

By that time your enmity should be cemented, so just keep spreading the combo and using flash in between every 2-3 combos so you can utilize the blind. This all depends on the DD's you have though. Some are better than others so you might need to focus on the first mob a little more than at other times.

One big thing with multiple adds is that you can just tab through as you do the combos and check the enmity meter. If you're completely full then you're good for now. Always check it to see where stand and which mob you need to focus on. Don't break sleep though, thats just extra dmg that you have to take.