Luca --
Since you provided this additional documentation about your problem, I now agree with Brian that it is pretty simple to assign resources the way you need to assign them to a single task. Here is what you will need to do:
1. Display the Gantt Chart view and then double-click the task in question.
2. In the Task Information dialog, click the Advanced tab.
3. On the Advanced page of the dialog, click the Task Type pick list and select the Fixed Duration task type.
4. Click the OK button.
5. For the selected task, enter a duration value measured in days in the Duration field.
The first five steps will "lock" the Duration of the task so that Microsoft Project will not change the Duration while you are assigning resources to the task.
6. Leave the task selected, then right-click anywhere in the white part of the Gantt Chart pane and select the Show Split item on the shortcut menu.
7. In the Task Form pane, select the names of each of the three resources you want to assign to the task.
8. In the Work column for each of the three resources, enter the amount of work measured in hours that you want to assign for each resource.
9. In the Task Form pane, click the OK button.
Microsoft Project will assign the resources to the task with the amount of work you specified for each of them. Hope this helps.
