Former Plaid Cymru leader Adam Price is unlikely to be re-elected in next year's Senedd election.
Cefin Campbell and Nerys Evans are the top two candidates the party will put forward for the new seat, while Price is third, the BBC has been told.
Plaid Cymru's rules aim for gender equality, meaning the second name on each regional list must be a woman.
The party would expect to win two of the six Sir Gaerfyrddin seats and face a challenge from Reform and Labour at the election in May 2026.
Price said: "I would like to thank the members that showed me support during this process and congratulate Cefin on topping the list."
Plaid's selection process is more competitive than Labour's or the Conservatives as the party is not giving its current MSs any preferential treatment and is prioritising gender balance in how it determines the order of candidates.
In Sir Gaerfyrddin, this means that if a woman tops the local members' selection, another woman must also be placed second on the list of up to eight candidates on the ballot.
