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I would recommend reading up on copying classes in C++ here's a good description. The problem seems to be on this line:

PointCloudSubscriber pcs[] = { pc1, pc2, pc3 };

You've instantiated the pc1 ... pc3 classes using their constructors which in turn has subscribed them to the cloud topics. But the line above then creates three new class instances using the default C++ class assignment behaviour described here.

So now you have 6 instances of the PointCloudSubscriber class three which are receiving cloud messages and a different three which are having their size printed out on the screen (which are always zero because they are not receiving any messages).

You could instantiate the classes directly in the array to avoid this problem like this:

PointCloudSubscriber pcs[3];
pcs[0] = PointCloudSubscriber(nh, "/kinect1/sd/points", queue_size);
pcs[1] = PointCloudSubscriber(nh, "/kinect2/sd/points", queue_size);
pcs[2] = PointCloudSubscriber(nh, "/kinect3/sd/points", queue_size);

Now you'll only have three class instances and it should start behaving the way you expect.