You are correct, you can leave the EveryStep script player running in the background at a frequency of every 3 hours, but it was not designed to start and stop monitoring a script on demand through the command line.
For regularly scheduled monitoring we recommend that you test a 30 day free trial at https://userauth.dotcom-monitor.com/Account/FreeTrialSignUp where you can upload the script to a Dotcom-Monitor cloud account.