What Program Allows Me To Have A Main Computer That A Few Others Can Access And Change Documents At Any Time

Main computer is the office computer. Its a desktop that a person is constantly on and doing stuff. There are 3 others that need the same information and documents at any given time. These 3 people have their laptops that are constantly switching wifi hotspots. They need to change information sometimes, and need access to all of it any time. The way its setup now is 1 person is on the main computer, but when one of the three need to access it, they remotely log in and take control. Only one person can access the computer at a time. How do I set it up so all four of these people can access and change the files at the same time if they want? The main computer is being used constantly, but someone always needs a document or file while the main person is working on it. So it slows production when someone logs in.