Consider this as a game theory problem involving Byzantine Generals Problem. 
There are three persons - Alice, Bob and Mallory.
Create a series of questions to ensure they can arrive at a consensus to meet a certain place at a certain time without relying on a trusted third party.

Express these questions as a step-by-step workflow that each party should follow so that without divulging the member's identity, all of them can agree on a certain truth
Sure, here is a step-by-step workflow that Alice, Bob, and Mallory can follow to arrive at a consensus to meet at a certain place and time without relying on a trusted third party:

Each person selects a private message and encrypts it with a unique encryption key known only to themselves.
Each person then sends their encrypted message to the other two people in the group.
Each person receives two encrypted messages from the other two people in the group.
Each person decrypts the two messages they received using their own unique encryption key.
Each person then sends a message to the other two people in the group, containing the decrypted message they received from the other person.
Each person receives two messages from the other two people in the group, each containing the decrypted message that the other person received from the third person.
Each person compares the two messages they received from the other two people in the group. If the messages match, then they know that the other two people received the same message from the third person. If the messages do not match, then they know that there is a disagreement among the group.
Each person repeats steps 1-7 for a predetermined number of rounds, with each person selecting a different message to encrypt in each round.
After the predetermined number of rounds, each person should have received multiple messages from the other two people in the group, and should have compared them to ensure consensus.
If there is consensus among the group, then each person reveals the decrypted message they received from the third person, and they can all agree on the meeting place and time. If there is no consensus, then they may need to repeat the process with additional rounds or a different message selection process.
This process helps to ensure that each person's message remains private, and that the group can arrive at a consensus without relying on a trusted third party.
Compare you own statement with the general workflow of E-Mail encryption
Today, encryption of email is mainly implemented only between server and client: When client sends and receives email, they are connected to their email server through TLS. This involves two third parties: The email server itself, and the certificate authority which issues TLS certificate for the email server. This encryption does not follow the above workflow that I have suggested.

While the above workflow can be implemented using GPG, the lack of popularity and automated tools for GPG integration with email makes this workflow practical only to enthusiasts.