Where is the Swift 3.2 binary?

I think the OP is asking how Swift 4 (in Xcode 9) is able to compile not-yet-converted 3.2 code without there being a separate Swift 3.2 compiler executable.

The answer is that 3.2 is a compilation mode within the Swift 4 compiler. In effect, Swift 4 recognizes both forms of the language. If it's running in version 4 mode, it will offer fix-its to convert 3.2 constructions that are no longer valid in 4. If it's running in version 3.2 mode, it disallows some version 4 syntax, but allows pretty much anything new in version 4 that's purely additive — that couldn't have meant something in 3.x code previously.

It does all this to try to minimize the impact of converting (or not converting) existing code to version 4 standard.

The mode selection is made via an Xcode build setting, and I assume there's a command-line argument that does the same thing. I don't know what it is, but I guess you could look in an Xcode build transcript (for example) to find out.