How to set a solution file to run in windows application.

Question

I made a solution file in windows application in which i have 4 projects
 
1-Administrator
2-Distributor
3-Manufacture 
4-Sub Distributor
 
when i run the solution then default Administrator project is run.
each project have its own "Application.myapp"
so i want to know that what can i do to run Distributor project and simultaneously run for Manufacture and Sub Distributor.