That's the obvious way to do it - you're not switching the earth there.
You could do it by always sending IN or OUT to all solenoid packs, and use the selector switch to ground the solenoid pack you want to work, BUT,
- That's a hell of a lot of extra wire for the sake of three relays
- You open up the possibility of a huge malfunction if a solenoid gets earthed some other way, such as water, a stray bit of wire etc.