There is no way to setup a cron job in the control panel for every few days to restart the VPS, however if the VPS supports it you could setup a cronjob to run inside the VPS to run the operation "shutdown -r".
Is there any reason for you needing to restart the VPS every 3 days?
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Restart a VPS and restart the OS used in the VPS are not the same job. If you want restart the VPS based in Virtuozzo enter VZPP ans simply click the Restart option: this is easy and secure. To restart the OS you can do it by the CP you are using: cPanel, Plesk or Direct Admin are secure for that task. You also can restart the OS by SSH: shutdown -r now But note: in some VPS environments, depending of the OS and CP used, if you does a restart by command line your CP may be unavailable...