I just want to add why the whole thing (WiFi) is a problem and why going for a "cluster" makes zero sense for the setup. I do not know the motivation behind having it as a cluster (it will kill the hardware, especially drives over time, but that aside), but one of the typical reasons would be to use "high availablity" ... however that will get your nodes rebooting anytime there's a problem with quorum (i.e. intra-cluster comm problems).
When you look at official docs [1] - the key part is:
Network Requirements
The Proxmox VE cluster stack requires a reliable network with latencies under 5 milliseconds (LAN performance) between all nodes to operate stably. While on setups with a small node count a network with higher latencies may work, this is not guaranteed and gets rather unlikely with more than three nodes and latencies above around 10 ms.
The network should not be used heavily by other members, as while corosync does not uses much bandwidth it is sensitive to latency jitters; ideally corosync runs on its own physically separated network. Especially do not use a shared network for corosync and storage [...]
So for that reason ... I glanced up again at your last answer ... I never suggested virt-manager just to be clear.
I wanted to help you make it work properly assuming this is some sort of school project. If you are able to run Ubuntu on those laptops, they should be able to run Debian (i.e. PVE), I believe the only issue you had was you were not connected on the wireless network, that's all. Once you get wired connectivity, directly installed PVE on each laptop should work for you.
Also to be clear - nothing in this setup (laptops) is production, but WiFi (for the reasons above) is something I would not even troubleshoot.
I can imagine someone at home using 3 old laptops (ideally with built-in ethernet cards) for this as a learning project.
The second issue - you mentioned it's meant for S3-like storage ... so again problem with the network is ... your S3 (or migration/replication even within PVE) can saturated that one network. Which is why a production setup would use separate NICs ... if you want to press on, you may want to have 2 dongles for each laptop. Keep in mind your S3 performance will be at 1GBps max anyhow, but I think you know that.
Don't get me wrong, all this is great learning experience, even bridge, virt-*, iptables, etc. but if you will be the one then called when things break down your internship will be miserable going forward... boss is an IT person? Maybe the best strategy is to impress him with your due dilligence and revert back with an (in)feasibility study...
[1]
https://pve.proxmox.com/wiki/Cluster_Manager