Let's say I am a law-abiding German citizen, but I have 3 BTC, that have been swapped from XMR and I can't prove the origin of them at all. Then after holding for 1 year, I cash them out tax free to my private bank account via coinbase or something.
Can the fiscal authorities (Finanzamt) then force me to prove the origins of my BTC? What if I don't do that. Would I be doing a crime, even if I am not money laundering and there is no further evidence in that direction? What crime would that be? Could they take my money?
In practice the fiscal authorities are likely the last in line to ask for a proof of source of funds. Both exchanges and especially your bank are likely to ask for the source of your funds first. Both of which can freeze your assets until the issue has been resolved, though banks seem more likely to simply close up your account and stop doing business with you, presumably because that will give the bank less of a legal headache than preventing you from accessing your money.
Whether you throw up any red flags and get asked for proof of source of funds will really depend on the sums involved and your overall situation. If you're a student with little income and suddenly receive a six digit sum you'll not only give your bank's compliance officer a heart attack but will likely be up for questioning as well. If you have a good middle class income your bank is unlikely to even notice if you receive a few additional hundreds or thousands of Euros every now and then.
As far as source of funds is concerned -- Even if you have no records of the swap from BTC to XMR, or of what happened with the XMR afterwards, it might be helpful if you at least have proof that you held those BTC in the past, ie. old wallet files. If you can proof at least part of your transaction chain, for example that you bought (a reasonable amount of) BTC that you then traded for XMR it may not be perfect but at least something. (assuming you get asked for proof of source of funds in the first place)
Yes you have to provide how the funds came from and since its 3 BTC it better to report it by yourself before they ask, if you bought years before now it gave huge returns which converted into 3BTC or whatever you have to pay the capital gain taxes then you will be safe and you know tax evasion is a crime as well.
If you hold your assets for more than a year there's no capital gain taxes to be paid on cryptocurrency profits in Germany.