That would depend on the impedance of the iPad output and the impedance of the headphones. It can be shown that the volume on each headset would remain almost unchanged, act as described or that adding extra headphones could cause a drastic drop in volume, depending of the impedances involved. Having the volume drop by exactly 1/2 or 1/3 with 2 or 3 headphones would be an unusual situation. It's also possible that driving 3 sets of headphones would damage the iPad.