MySQL's AND clause will only work when ALL of the criteria are met. What you're looking for is OR. In the format that you provided:
UPDATE participantes SET confirmado = 1 WHERE id = 19 OR id = 20 OR id = 21;
Although, the above-noted IN (19, 20, 21) would be better for this specific use case.
Your original query was trying to look for a single row where id was simultaneously 19, 20, and 21, which would never happen.