Trouble with assigning multiple resources to a single activity in P6v7

I am trying to load multiple resources on a single activity and assign estimated hours to each resource. Let's say I have an activity that is 20 days in duration, resource 1 has 100 hours, resource 2 has 80 hours and resource 3 has 50 hours. When I assign the budgeted hours to the first resource, the hours will spread across the duration and calculate the budgeted units / time but for resources 2 and 3 the hours will not spread. If I change the activity duration to 0 days then change back to 20 days, the budgets will spread