The key advantage of 3PC over 2PC is that it unblocks one of 2PC's blocking scenarios. When the coordinator crashes after sending PreCommit, participants can proceed to commit without waiting for recovery.

2PC blocking scenario:

1. Coordinator sends Prepare to all participants
2. All participants vote Yes
3. Coordinator crashes before sending Commit
4. Participants are blocked: they don't know if the decision was Commit or Abort
5. Participants must wait for coordinator recovery

3PC non-blocking scenario:

1. Coordinator sends CanCommit to all participants
2. All participants vote Yes
3. Coordinator sends PreCommit to all participants
4. All participants acknowledge PreCommit and enter PRE_COMMITTED state
5. Coordinator crashes before sending DoCommit
6. Participants are not blocked: they know everyone voted Yes, so they can commit

Recovery rules for participants in PRE_COMMITTED state:

function onTimeout() {
    if (state === PRE_COMMITTED) {
        // We know everyone voted Yes, so we can commit
        commit();
        broadcast("I have committed");
    }
}

Example test:

Request:  {"type": "txn_begin", "msg_id": 1, "participants": ["p1", "p2"], "crash_coordinator_after": "pre_commit", "participant_timeout_ms": 2000}
Response: {"type": "txn_begin_ok", "in_reply_to": 1, "txn_id": "txn42"}

// After timeout, participants commit without coordinator:
{"type": "have_committed", "src": "p1", "txn_id": "txn42", "recovery": "timeout"}
{"type": "have_committed", "src": "p2", "txn_id": "txn42", "recovery": "timeout"}