I think 3 is enough with for a 6ft tank, but not enough for an 8ft tank. But honestly, I think you can strategically place your corals around the 3 halides.
Are the halides going to be complimented with other lights like T5's, or is that all the light you plan to have? Because for me, I like to have light along the entire length of the tank.
125 gal. reef w sump/refugium & 8-bulb 640w T5's
45 gal fresh w/plants + CPF lighting