custom picklist based on another application

custom picklist based on another application

Iamb trying to build a few custom applications:
Companies
Users
Inventory


Id like to build a master application that draws all of them together, called Admin

I want to enter a list of companies
Then enter a user, and select his company
Then enter the Inventory assigned to the user.

I want to build the application so that I select the company when enter the user, but later on I want to be able to get a list of users in each company.

Then I want to be able to generate a dropdown menu/lookup field to see inventory in the company, users in the company, and a user's inventory.

What's the best way to do this? Is it possible with 3 separate applications? If not can it be done with just one application and a bunch of views/forms?