One to let water in, one to let air out, and one to let air in. In usage you open the water valve to allow water in. Then you slowly open the air outlet valve to allow the air holding water out of the VBT out. If too much water is let in by letting out to much air you can add more air with the air inlet valve. Once neutral bouyancy is obtained close all the valves.
Waterline Is where the water meets the hull of a vessal when it floating or positively bouyant.
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