Remote access server separately for each user, how to organize?

Question

Windows

There is a computer, Windows 7 x64 is installed on it. Created 4 accounts:
admin - admin and 3 normal - user1, user2, user3
On the accounts of ordinary users, the application prog.exe is running.
Question: how can I implement the ability to send keystrokes to all these applications from the admin account?
It should look something like this:
- go to user1, run prog.exe, start-switch user
- go to user2, run prog.exe, start-switch user
- go to user3, run prog.exe, start-switch user
- go to admin and send keystrokes to these processes
I tried VNC, but I understand that it makes the server of the machine as a whole and does not work with individual users.
upd1. Now it is implemented as follows. A wmvare virtual machine is installed, 3 windows clones are created there and a vnc connection is configured. Thus, everything works as it should, but the resource consumption for running prog.exe is unreasonably large.
upd2. Prog.exe is protected and you cannot run 2 or more processes at the same time.

Answer 1

Terminal Services allows you to connect to sessions, only the OS needs a server one.

Answer 2

We put Windows 7 in a virtual machine or on a real one.
We make a terminal server out of it. There is in google. There are a couple of files that need to be replaced.
create three profiles.
connect from a host or another computer. (so that 3 windows were mstsc).
Well, then you do what you need to do there.
I see it that way.