Hi and welcome to the forum…
- Do you mean you want the FP3 not the FP3+?
You can not use both as the bumper or the case each fits to the phone not on top of each other.
The bumper does do something but I would argue the case is better, well I wanted the case and I’m not sure you can buy the bumper but apparently it does come with the FP3
So you can show of the innards a bit with the FP3 as the back cover is transparent and the bumper doesn’t cover it.
However note that Fairphone state, “The Fairphone 3 ships with a protective bumper. The Fairphone 3+ does not need one.” so I wonder why that is ??