Your question can be easily answered with some calculations. The answer appears to be YES, all flights are priced in euros and choosing to pay in another currency incurs a DCC scam markup of around 5-6%.

Interestingly, A3's Russian ruble DCC rate is around 15% *cheaper* than the actual exchange rate right now, but I think that nobody with a RUB-denominated card is going to be able to use it on A3's website right now, so anyone who wanted to try this would be gambling that the Mastercard / Visa rate from their card currency to rubles remains in their favour at the time of transaction posting (unless you use a product like Curve or Revolut which fixes the exchange rate at the time of authorisation).

This appears not to be the case for A3. Anyway, you can only change currency with BA on phone bookings, which incurs a £15 fee in the UK, or a similar amount elsewhere, otherwise it's based on the currency of your flight origin country (not always the local currency, for example BA Romania flights are in euros).

I haven't flown Air Canada for about 8 years but back then, I recall that their website did allow you to select a wide range of currencies to pay in regardless of your flight origin country, and the IATA exchange rate would be used.

Also Norwegian, when I last booked with them in 2019, was allowing you to pay for any flight in any of their supported currencies (a selection of around 10-15 currencies, used in the countries they flew to). They did not use the IATA exchange rates, but it was not DCC either. The rates were somewhat random and by experimenting, you could sometimes save over 10%, but IIRC you had to redo the search from the beginning. Flights tended to be cheapest in GBP and/or SEK, even for flights that did not involve the UK or Sweden, but given the cheap prices, spending 15 minutes to end up saving £1/€1/10kr was not always worthwhile plus there was a risk that the exchange rate would move against you by the time the transaction processed.

One option to pay Norwegian was in Argentinian pesos. I recall that for one flight I was looking at the peso price was nearly half the price in the other currencies - now the blue market rate is around half of the official rate, but I wasn't sure if it was some sort of error or what rate my card would end up using, so I didn't risk it, after all that flight was only £30 or so. This is probably similar to the ruble situation I mentioned above.

The scam is that A3 charges a 5-6% fee for this conversion, and people may not even notice, for example Knobbgb says the rate always seemed about right. Had I booked some A3 flights recently, I might also have fallen victim to this as I assumed flights out of LHR would have a base price in GBP - but it is in euros. This also explains why OTAs such as Expedia pricing in GBP, at the IATA exchange rate, are cheaper than A3 direct - ignoring discount OTAs which are unreliable.

I have bank cards which do not charge any conversion fees to pay in a different currency, so I would use these to buy a flight priced in euros. I would only be subject to the Mastercard markup of typically -0.25% to +0.5%, or the Visa markup of typically +0.5% to +1%. (Well, I do have a card which charges 1.95% for non local transactions, but also pays 2.4% cashback on transactions in a user-selected category, with the option to select foreign transactions as the qualifying category).

This is not DCC. DCC is when the merchant determines the exchange rate. If your card is in SEK but you buy a product in GBP, the merchant can charge you their actual price in GBP. This will be converted by Visa/MC/Amex etc at a slightly worse rate than interbank, with MC very occasionally slightly better (note that China Unionpay doesn't have any markup and doesn't allow forex fees), and your bank may apply a further fee.

Or the merchant can charge you in SEK, and usually they would add a DCC markup of 4-5% when calculating how much SEK to charge you. I don't think any Swedish banks charge more than 3% forex fees, so it's always better to decline DCC, unless you think the SEK is going to drop vs GBP by more than 3% in the next 24 hours - then you would "lock in" the 5% worse DCC rate now to avoid paying 5% more when your transaction is processed after the SEK drops.

Well, this varies by card issuer. Some only charge the fee when transacting in a different currency and don't care about the country of processing.