Screenshot every 3 seconds?
At the moment, MobiControl does not have the ability to automate and schedule script commands.
You can schedule script commands in somewhat of a roundabout way if you really need to, although the solution is somewhat impractical for this situation. What I've done in the past to accomplish this is create a Package containing just a post-install script with the script that I want regularly applied. I then add the Package to a Profile with the option to re-install the packages on every update schedule. I then will tweak the device's update schedule configuration based on how frequently I want the package applied. It's unpractical for this particular example however, as I don't think you'd want the devices checking into the server every 3 seconds.
I have done this several times with a sync file rule. Set the rule to download an empty txt file to the devices and to always execute the post script. You can set the time schedule as you need. Hope it helps.