The plumber did a really great job providing a lot of exposed baseboard throughout my 1st floor, but in doing so had to route many different vertical/horizontal turns and ups/downs because my house is on a slab. I don't think he secured the pipes enough because I get extreme water hammer when only 1 of the 3 zones shuts off. Since this zone is my main living area zone, can I manually open and keep open the Honeywell zone valve which I'm sure closes too fast causing the banging? Even if I call for heat in the other 2 zones, garage and 2nd foor, I know the zone held open will also heat. This should not be a problem as it is heated more than the other two. Is my thinking correct?

This sounds like a lot of popping and cracking of a hot water base board heating system rather than water hammer as defined by plumbers. The popping and cracking is caused by the rapid heating and cooling of the pipes as opposed to rapid action of a valve. The rapid heating and cooling is caused by air in the system. Recommend you attach a garden hose to the system and systematically bleed the system of air. Let water run until a steady flow of water comes out.