Sorry but your scenario isn't quite clear. I sounds like you have a resource that only works one day a week and when assigned to a three day task, you expect the task to expand to three weeks. Is that right? If not, please clarify what you need.
If my interpretation is correct, this is what it might look like. The resource calendar is customized so Monday is the only working day.
Then a three day task is created. To help visualize what happens the Task Usage view shows timescaled data for 3 consecutive days before the resource is assigned. Note, the Ignore Resource Calendar field is also shown to indicate the task will follow the resource's work calendar.
Now the resource is assigned at 100%. The result is that the task duration expands to cover three Monday's work.
Hope this helps.
John