I use 2 units of 3 and i dont think i would ever use them as 6. I normally deep strike out of LOS near an objective normally if i can. The fear factor alone means that the other army will hesitate before moving anywhere within 15 inches or so of the flamers. Sometimes to earn there points they dont actually need to do any damage. And a unit of 3 is so easy to hide behind cover.
Yes having a unit of 3 does help when doing suicide attacks but I tend to go for the safer option and use them later on where they just clean up and kill taht annoying unit sitting on the objective with a good cover save.
In the end the way i see it is that if they are in the open are getting shot whether there are 3 or 6 of them they will most likely all die or nearly all die so better to present 2 targets than 1.