Как составить запрос MySQL?
просто дописать джойны всех трёх таблиц
Нашел решение сам:

SELECT
    appeals.id, appeals.text, appeals.answer, appeals.joins, appeals.supports, appeals.dislikes, receivers.code, receivers.name_ru,
    EXISTS(SELECT * FROM joins j WHERE j.appealId = appeals.id and j.autorId = 1) AS isJoined,
    EXISTS(SELECT * FROM supports s WHERE s.appealId = appeals.id and s.autorId = 1) AS isSupported,
    EXISTS(SELECT * FROM dislikes d WHERE d.appealId = appeals.id and d.autorId = 1) AS isDisliked
FROM appeals
	LEFT JOIN receivers ON receivers.id = appeals.toId
WHERE appeals.lang = "ru"
ORDER BY appeals.id DESC;