I’m not sure… Hedgehogs seem to like going around the perimeter of the garden, so if it’s in the middle of things it might take longer for them to find it. Mine are clustered at the bottom of the garden, perhaps eighteen inches from the wall of the shed, because I didn’t want the rain from the shed roof coming down directly on the houses! The hogs often wander between the shed and the houses, but they also go between the houses and come round the front to access them.
But yes, if you have the space and the resources, I’m sure more houses would be appreciated. If you have seen three hogs you might well have a few more, as they can look very similar and can often be mistaken for each other. Additionally, they can move into a new nest during hibernation, so it’s good to have some spare houses if you can manage them.
How bad is the flooding? If it’s just a bit soggy and not actually prone to disappearing under inches of water, it might still be ok to put the houses there, just raised up on bricks or whatever.