Are the 3 installed PHP versions 5.2, 5.7 and 7.0 required? Only one I'm interested in 7.0

they are required only if you use php-5.0-5.2 or php-5.3-5.6. If you're using only php-7.0, you can safely uninstall them by running unins000.exe in their directories

I assumed that was the case; but, didn't want to screw up the whole phpEd installation.

you can select target php version on project properties -> parser properties page. The drop down has 4 chooses: 4.x, 5.0-5.2, 5.3-5.6 and 7.x. These versions select what php parser will check your sources and what code completion for standard functions/classes will appear. This is a built-in functionality that does not depend on php installed on your system.
In case if your project Run Target is "Srv Local", IDE runs php.exe or php-cgo.exe configured on Tools->Settings-> PHP CLI or SRV web server pages respectively. This is the only place where those 3 versions of PHP appear and used.