What is the difference between the Apple Watch series 3 with that red dot and another series 3 without the red dot

The Apple Watch Series 3 with the red dot on the digital crown is the GPS + Cellular version. The one without is just the GPS version.

I begin to realise that with cellular you can be away from the phone but the notifications can still come through plus you could reply. However with no cellular you need to always connect via Bluetooth to the phone to get the notifications and replies.

There are a few other caveats, you can look at them here, How to use your Apple Watch without your iPhone nearby - Apple Support