Performance at finding all foreigns with 3 one-to-many relation
Instead of doing 3 separate joins, you could accomplish the same thing by simply using an OR clause (the execution time seems to be ~1-2ms):
SELECT DISTINCT u.id
FROM user u
JOIN translation_revision t1
    ON u.id = t1.translator_id OR u.id = t1.proofreader_id OR u.id = t1.reviewer_id
GROUP BY u.id   
LIMIT 30

Other ideas would be to denormalize your data and keep some additional info about your users roles.