I would recommend going with five.
Three will work, if you prefer.

With a predominantly pvp oriented group of five toons...
I'd look hard at 4x Shaman, 1x Druid (dual-spec).

Have the Druid go Boomkin (and never drop stance for healing), in pvp.
That gets you 13% additional damage on the other toons.
Which is probably a fair bit more total damage from 5 toons (even not managing Eclipse) then 5 shammies would produce.
You can approximate Eclipse with 3-4x Wrath, 3x Starfire chains, for roughly half the effect of Eclipse now.
And Eclipse is going to be far easier to manage in Cataclysm.
Also, the big plus... Dual-spec the Druid to Feral.
Which gets you a strong tank for PvE play... and badges (heroics) are faster PvP gearing then grinding battlegrounds.

Plus you can then run PvE for variety and such.