Hi Jon. 3 times per hour is like the minium requirement, but in some cases very useful if for example extra CO2 is provided.
Generally the ideal exchange would be every five minutes. So 3 times per hour would be then every 20 mins. And if you've got a strong extractor fan which can change the full tent volume in couple of minutes then every twenty minutes wont be too bad if you've got something to push new air inside the tent for a least a bit. Thinking more about it even a human could almost spent about twenty mins inside the 3m3 tent without constant air flow.
Like that you just prob get most out of extra CO2 supply.
Folks seem to be using mostly passive intake flow nowadays, so that basicly requires a constantly running extraction fan. For two things to keep the intake flowing and drawing moist air out.
If the intake is passive and you shut down the extraction fan then the air inside the tent move at all. In that way the plants prob start sweating after 10mins which then creates way too much condensation.
Ye this true. It does not harm the plants even if the tent walls are dented inwards due to hard negative pressure