SQL fiddle
CREATE TABLE Coordinate
( X int, Y int,Z int, Disabled bit, OccupiedId int)
INSERT INTO Coordinate VALUES (1,1,1, 1, NULL)
INSERT INTO Coordinate VALUES (1,1,2, 0, NULL)
INSERT INTO Coordinate VALUES (1,1,3, 0, NULL)
INSERT INTO Coordinate VALUES (1,1,4, 0, NULL)
INSERT INTO Coordinate VALUES (1,2,1, 0, NULL)
INSERT INTO Coordinate VALUES (1,2,2, 0, NULL)
INSERT INTO Coordinate VALUES (1,2,3, 0, 123)
INSERT INTO Coordinate VALUES (1,2,4, 0, NULL)
INSERT INTO Coordinate VALUES (1,2,5, 1, NULL)
SELECT X, Z, MIN(Y) AS FirstFreePosition
FROM Coordinate
WHERE Disabled = 0 AND OccupiedId IS NULL
GROUP BY X, Z
OR -- if you need the unavailable combinations too, then something like this:
SELECT X, Z, MIN(CASE
WHEN Disabled = 1 OR OccupiedId IS NOT NULL
THEN 1000 --a big number
ELSE Y END) AS FirstFreePosition
FROM Coordinate
GROUP BY X, Z