The 3€/month is a single thread (not even a core), and it's not even dedicated. The lowest dedicated instance is 2 vCPU (1 core) for 23€/month with 8GB or RAM. That's 276€/year.
8 of those instances is equivalent to an 8 core (16 thread) chip with 64GB of memory. Using, e.g., Supermicro AMD EPYC 3251 + parts, you could build an entire server for less than 1/2 that price. You could have twice the compute for the same price the first year. Servers easily last 3 years minimum, so Hetzner has a 6x markup at the low end.
Admittedly, building your own server doesn't include transit, but transit is cheap. Assuming you have a fast home connection (a big if for many people), the hard part is access to static IP addresses. There are dedicated services that lease IPs over a VPN tunnel, or you can setup your own VPN into, e.g., Hetzner or Vultr using a handful of the cheapest instances. (Not sure about Hetzner, but Vultr let's you lease and attach 2 additional IPs to the same instance for $2/month/IPv4 address.)
Now, the above setup is probably far too much trouble for most people, and it has many downsides[1]. But the point is that if we're just talking about CPU time, not even Hetzner or some other cheap VPS provider can come close to the cost of running your own equipment.
[1] Power and cooling are two downsides, but the 3251 has a TDP of 50W. Granted, it doesn't have the highest clock speeds, but there's a reason cloud providers don't normally advertise Ghz. Anyhow, rather than build 2x 3251's you could build a single 16 core rig using a higher-clocked and newer EPYC or Xeon for about the same money (definitely at or under 2200€ all-in).