There are two types of 3-Way Ball Valves: the Diverter Port and the Multi-Port.  What's the difference?  Well, it may not be as complicated as you think.  Let's start with the Diverter Port.  The Tru-Flo 3-Way Diverter Port Ball Valve is designed to have the center port act as the common port.  This means that the center port must be open at all times.  You can use the L-Port Diverter Port to go from center to left (Fig. 1-1 & Fig.1-2) or to go from center to right (Fig. 1-3).  The T-Port Diverter Port can do the same configurations as the L-Port but can also have all three ports opened at once. 

The second type of 3-Way Ball Valve is the Multi-Port.  It is primarily used when an application requires a 3-Way Ball Valve to close the center port and have the flow go straight through the valve (Fig. 2-1).  The Multi-Port is designed to have a positive seal on all ports, therefore you do not need a common port. With the L-Port Multi-Port you can go from center to left (Fig. 2-3) or center to right.  The T-Port Multi-Port can do the same configurations as the L-Port but can go straight through / left to right (Fig. 2-2) and can also have all three ports opened at once.