There are 1 valve systems (with backflow preventers), 2 valve systems, and 3 valve systems.
All keep the antifreeze out of the water heater. That is the only reason they exist. If you blow out the lines, you don't need a bypass.
I don't like the 1 valve system because the backflow preventer can fail. Mine has. I have a bypass valve to put in instead, but haven't had the motivation to work in the small space behind the water heater yet.
I like the 2 valve system best. The two valves being bypass valves, one on the hot water line, and one on the cold water line.
I don't see the need for the third valve in the 3 valve system. Instead of 3 shutoff valves with 2 T fittings, 2 bypass valves do the same thing.
