The shield is grounded of course, but what you need to find out is what the other two do. They either represent an "in" and an "out" of the humbucker, or the shield is doubling as the "in", one of the wires is the "out" and therefore hot, and the other wire is the coil junction used for coil splitting. If you have an ohms meter, it will read like this:
A) Shield to one of the wires is "x"(roughly between 8k and 21k) while shield to the other wire is roughly half of "x"(roughly between 4k and 11k. In that case you have a shield, a hot, and a coil junction.
B) Shield has no connection to either wire and the two wires read roughly between 8k and 21k. In that case you have a shield, a hot, and a ground.(or in and out) The hot and ground can be reversed if the pickup is out of phase with the others.
If you have no meter, it will come out like this:
A) Shield to ground, then hook one of the wires to hot, and see if you get signal. If so, then disconnect that wire and hook the other wire to hot. It would be twice the volume or half the volume depending on which one you got first. Then you know you have a hot and a coil junction.
B) Shield to ground, then hook one wire up, and you get nothing. Touch the other to ground and all of a sudden the signal is there. Then you know you have a hot and ground.