My assumption is that those hostnames used to be hosted at the IP that you are now using, and that there's some service on some forgotten server which is dutifully trying to revalidate control over its names every 3 hours, even though it won't work until they update their DNS records or otherwise fix their configuration.
Such "zombie" clients that just retry forever, hoping against hope that one day they'll be configured correctly but in the meantime just keep wasting resources failing, are unfortunately much more common out there than one would hope. They once said that around 80% of HTTP validation attempts fail, and a lot of them are these forgotten clients that aren't smart enough to give up after retrying for months (or years).
I agree with the previous poster that there really isn't anything for you to do. If you're ambitious, and generous, and want to try to figure out how to contact the network operations of the domain that's failing, I suppose you could give it a shot. But if a few requests, every 3 hours, is the worst noise in your log files to deal with, then you're doing pretty well. If it really bugs you, then you might be able to get a different IP from your hosting provider, but there's no guarantee that any other IP would get any less random traffic from a prior owner.