That's the only way to know. IMO it's not the 3 cm but the total effect on air temp inside the case. For example many PC power supplies exhaust hot air. If you position the PC just so you might see this hot air get sucked into what looks like in your picture intake vents.
It should be fine but only by checking temps will you know.
I use Speccy but on some PCs that misreports and I'm using something else.
Hey, I recently mover my PC a little bit and have it now in a little bit of a tight spot between the fan holes for the air circulation and the wall is about 3 cm. Will it be enough?
If someone isn't sure about the size of the holes for the air curculation here's a link https://imgur.com/RIpCSh7
Please don't comment on the dirtiness of the machine.

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