Why did the Planner put all the tasks for my paper into one week, when it's not due for 3 weeks?

Probably because it's also taking into consideration your other assignments.

You can adjust the plan by either:

1. going to the Add/Reschedule page and setting the deadlines you want for the paper steps and checking off the "Fixed Date" box and then re-scheduling, or
2. you can just move them with the "Re-assign to Week" drop down boxes in the All Checklists page.