@David9258 This is just a discussion forum for peers. Your case is not being litigated here, so there's no need to call in witnesses. You also can't post the full name of another member here, so your host's name will most likely be censored.
You might want to familiarize yourself with the actual terms of the damage claim policy to better understand what's going on. You'll see, for example, that 3 days is well within the 14 days from your checkout that the host has to initiate a damage claim.
I see no reason that it should be considered bad practice to "mix Airbnb and a regular flat share," as long as the local law and the housing community permit it. But if other guests or residents happened to be sharing the facilities during your stay, any of them could have conceivably caused the damage, so that detail would be very relevant to your dispute.