turnaj-sim.js

let successes = 0;
let failures = 0;
for (let iter = 0; iter < 100000000; ++iter) {
    let counts = [3,0,0,0];

    while (counts.length > 1) {
        let loser = Math.round(Math.random() * counts.length);
        if (loser === 0) {
            break;
        }
        if (++counts[loser] >= 4) {
            counts.splice(loser, 1);
        }
    }

    if (counts.length > 1) ++failures;
    else ++successes;
}

console.log(`${successes} (${failures}) -> ${(100.0 * successes / (successes+failures)).toFixed(2)} %`);