FreeRTOS Tasks stack size on STM32L4

Hi all,

I am working on stm32l486 chip.

When I created (using CubeMx) two tasks with a stack size equal to 256 words. The project is running well:

osThreadDef(Task1, vTask1, osPriorityNormal, 0, 256);
Task1Handle = osThreadCreate(osThread(Task1), NULL);
 
osThreadDef(Task2, vTask2, osPriorityNormal, 0, 256);
Task2Handle = osThreadCreate(osThread(Task2), NULL);

The problem appears when I added a third task with a stack size equal to 256:

osThreadDef(Task3, vTask3, osPriorityNormal, 0, 256);
Task3Handle = osThreadCreate(osThread(Task3), NULL);

If I reduce the stack size of each Task to 128, the problem disappear.

Why I can't use 3 tasks with a stack size equal to 256 for each task ?