All the angles of your problem have been covered well by other posters.
If you are confident that your load is balanced and supply is balanced (phase-to-phase and phase-to-ground), you could use single wattmeter connected line to neutral/ground, and multiply it by 3 to get the total.
Otherwise, assuming you have to use more wattmeters.
One approach would be to use three wattmeters, each connected from a different phase to the same arbitrary neutral or ground point. Adding the three readings will give the correct results, regardless of whether the system is balanced and regardless of whether your measurement neutral matches the power supply neutral.
Bill has described the aptly-named 2 wattmeter method, which also gives correct results and saves you hardware and hookup time.
This 2 wattmeter method is surprisingly easy to derive (much easier to derive than to memorize imo):
S = Sum (Vk x Ik*) for k=a, b, c [Equation 1]
(where x in this post means vector multiplication, NOT cross-product, and post-script * means conjugate
P = Real{S}
The Ik in equation 1 are all entering terminal k (a, b, c).
The Vk in Equation 1 all need to be referenced to the same point, but it need not be a neutral!
We will choose phase A for our voltage reference. which results in the following:
Va = 0
Vb = Vba (where Vba = line to line voltage with + meter lead on b and - meter lead on a)
Vc= Vca (where Vca = line to line voltage with +meter lead on c and - meter lead on a)
Plug these values of Vk into equation 1:
S = 0*Ia* + Vba x Ib* + Vca x Ic
S = Vba x Ib* + Vca x Ic
P = Re{S} = Re{Vba x Ib* + Vca x Ic*}
P = Re{Vba x Ib*} + Re {Vca x Ic*} [equation 2]
Equation 2 tells us the total power is the sum of what is shown on two wattmeters:
1st wattmeter measures curent entering phase b with voltage connected b(+) to a(-)
2nd wattmeter measures curent entering phase c with voltage connected c(+) to a(-)
Once you understand the principle of the derivation, you should be able to jump directly to equation 2 without any intermediate steps.
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(2B)+(2B)' ?
