So, lately I've been reading about monitoring tools in unix forum.
For linux systems:
Monit - a simple monitoring utility
Check_MK
zabbix
icinga 2
Munin
For k3s pod(in rancher)
I've heard of healthchecks
There's grafana workload metrics there.
First I am unable to define what means that a pod is down. Because it's not necessary that always a pod is down only when it fully stops. Sometimes it is frozen, when logs stop coming. Or some service becomes unreachable. etc.
(Some service becomes unreachable is a valuable point of interest)
The need to monitor is just 3 servers so something like ELK stack will be an overkill. Also I'd prefer something that's open source and doesn't come with any cost in $$.