You will have to get 7 from a third-party owner. (EBay.) Or from the old software box if you still have it. (Maybe you saved it on an old HD.)
FCP7 is no longer sold by Apple.
Once you get FCP7, it might be a good idea to "hide" the current FCPX software from your computer in a folder somewhere, or wipe FCPX off your machine completely. Perhaps some folks in the FCP Studio forum can also help you.
Normally you have FCS 3 on your Mac first. The FCP X installer will create a folder inside the Applications folder called "Final Cut Sutido" and move all of your FCS 3 apps inside of that. You can do this manually if needed.
Move Final Cut Pro X to a different location (plus Motion 5 and Compressor 4 if you have them). Install FCS 3, and move all of those apps into a folder called "Final Cut Studio". Then move FCP X, Motion 5, Compressor 4 all back to their nomral location inside the Applications folder.