Hi all. I am from the UK and hold UK and Australian Citizenships. I am also eligible to become a citizen of another European country by descent. but have not yet exercised that right. Would becoming a citizen of a third country affect my Australian Citizenship?
No.
However either or both of the UK and the third country may have a problem - under either or both of their own law(s) - with you being a dual citizen each of the other, and with you being a (so to speak) "triple national".
Reason for looking at this is easier travel within Europe now the UK has left the EU.
You're possibly overthinking things? If "travel" in this case simply means recreational travel/ holidays,
then there's no immediately obvious benefit.
A Schengen nationality only really matters if you want to live long term/ permanently, or to work, in a Schengen country.
And even that is not reliable. Try being an
wanderarbeiter in Germany these days, and see how welcome you aren't.
Bear in mind also that you don't get to double-dip on consular support.
So, if you left Australia on, say for example, your French passport, and you get arrested in Portugal, then Australia cannot help you.