for the same values you can cheat and divide by the number of values (.7+.7+.7)/3 = 2.1/3 = .7 three 1.4 ohm loads would be 1.4 a ohm load
for different values (z1*z2)/(z1+z2) (for you it works out to .466 ohms) you can use this for as many as you like however do not run two speakers like this! one speaker would be getting about 66% of the power and the other would get 33%, and would sound like crap
Thanks for the help. The reson for the ? is I have a mt quad.7 and a mt dual .7, that i plan on running just 1 2500d for now. Would like to keep it at .5 or higher.