Devices.
Four iPhones. Three versions of iOS.
One app. And the iPod touch.
Four iPhones. Three versions of iOS.
One app. And the iPod touch.

iOS 4.3 to 6

iOS 5.1.1 and iOS 6

iOS 6
WA Classic runs on the iPod touch with the same iOS: the 3rd generation with iOS 4.3 to 5.1.1 and the 4th generation with iOS 4.3 to 5 take WA Classic for iOS 4 and 5, the 4th generation with iOS 6 and the 5th generation with iOS 6 take WA Classic for iOS 6. Everything works over Wi-Fi. The iPod touch 3rd generation has no microphone and no camera: calls and voice messages need a headset with a microphone.
Only the 32 and 64 GB models are a real 3rd generation. The 8 GB model sold as 3rd generation (model number MC086) is a 2nd generation inside and stops at iOS 4.2.1, so it can't run WA Classic.
WA Classic runs on iOS 4, 5 and 6. A version for iOS 7 to 10 follows later, in the flat look of WhatsApp 2.18, the last WhatsApp for iOS 7.

Any jailbreak for your iOS with Cydia.
For calls like a phone call. Cydia installs it with the app.
With WhatsApp. It stays the main device.
WA Classic is an unofficial client and not affiliated with, endorsed by or connected to WhatsApp or Meta. Using it may break WhatsApp's terms of service; your account could be banned. Messages are stored unencrypted on the computer that runs the bridge.
iPhone pictures on this page are drawings; the screenshots show made-up demo chats. WhatsApp is a trademark of WhatsApp LLC.