Não posso fazer isso. Tem que ser três exclusões separadas.
DELETE
FROM medewerkers
WHERE mnr = p_mnr;
DELETE
FROM UITVOERINGEN
WHERE cursus = p_mnr;
DELETE
FROM UITVOERINGEN
WHERE docent = p_mnr;
DELETE
FROM medewerkers
WHERE mnr = p_mnr;
DELETE
FROM UITVOERINGEN
WHERE cursus = p_mnr;
DELETE
FROM UITVOERINGEN
WHERE docent = p_mnr;