On your Hexnode MDM portal, upload the Android app APK you want to test.
Create a policy, set the test app as a mandatory app and assign the policy to the group.
The apps will be automatically pushed to all the devices in the group.
You can add app updates or different apps by editing the policy.
To revisit each step in detail,
You can register the devices easily by scanning a QR code. Follow this set by step guide on QR code enrollment
You may employ static or dynamic groups. If you have just the three devices, static groups would be fine. If you plan on expanding the test group with a lot more devices, a dynamic group would be the way to go.
Create simple static groups by following this help.