MSP has always had problems with splitting work among resources.  I would suggest 3 separate tasks.  You simply assign the duration with the number of hours in the Work column.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

You can click the "Assign Resources" button. From there if you know the total time of the task, you can assign percentages of the total task (which would give you a specific time period, 20, 15 and 5 days to each resource). You could go one step further and assign costs for each resource to get a ROM cost estimate for the task.

I disagree with either of the 2 suggestion answers above.  The problem seems to be that you are assigning resources using the system defaults that do not apply. You do not need to split the tasks into 3 to get the proper assignment. Using Assign resources box will always result in a surprise assignment.

First go to view --> more views --> task entry view.  Then right click in the bottom panel and select resource work.  Click on the task you need to assign resources to.  Click off the effort-driven flag and have the task type be fixed units. (click OK to update).  Now assign your resources stating how much work each resource should do (click OK to update).  The assignment will be made based on the availability on their calendars. Switching to resource schedule view in the bottom pane will allow you to refine the days the assignment is actually scheduled.

Creating assignments in the task entry view is the only way you are seeing what you are getting for an assignment.  If you don't get what you expect, you can tweek it here easily.

See my website at www.lehnertcs.com for more tips and tricks using MS Project.