MySQL 8.0

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SET @number_u:=2;
SET @max_cell:=3;

WITH 
cte1 AS (SELECT CASE WHEN @max_cell <= @number_u
                     THEN @max_cell
                     ELSE @number_u 
                     END AS number1),
cte2 AS (SELECT CASE WHEN @max_cell <= @number_u - number1
                     THEN @max_cell
                     ELSE @number_u - number1
                     END AS number2
         FROM cte1)
SELECT number1, number2, @number_u - number1 - number2 number3
FROM cte1
CROSS JOIN cte2;


number1
number2
number3


2
0
0


SET @number_u:=5;
SET @max_cell:=3;

WITH 
cte1 AS (SELECT CASE WHEN @max_cell <= @number_u
                     THEN @max_cell
                     ELSE @number_u 
                     END AS number1),
cte2 AS (SELECT CASE WHEN @max_cell <= @number_u - number1
                     THEN @max_cell
                     ELSE @number_u - number1
                     END AS number2
         FROM cte1)
SELECT number1, number2, @number_u - number1 - number2 number3
FROM cte1
CROSS JOIN cte2;


number1
number2
number3


3
2
0


SET @number_u:=15;
SET @max_cell:=3;

WITH 
cte1 AS (SELECT CASE WHEN @max_cell <= @number_u
                     THEN @max_cell
                     ELSE @number_u 
                     END AS number1),
cte2 AS (SELECT CASE WHEN @max_cell <= @number_u - number1
                     THEN @max_cell
                     ELSE @number_u - number1
                     END AS number2
         FROM cte1)
SELECT number1, number2, @number_u - number1 - number2 number3
FROM cte1
CROSS JOIN cte2;


number1
number2
number3


3
3
9