I would like to be able to charge my spare battery separately from the phone itself. I have found a universal charger that looks as though it could, physically, work: the pins seem to be adjustable and the pins are, in fact, blades, that should fit into the narror contact slots on the FP3 battery.
I guess, you should go for the optiong @AnotherElk has pointed to.
The charger you have linked to, says to be loading batteries with a voltage from 6.8 to 16.8. Since the Fairphone batterie has 3.8 only, I wouldn’t consider using this charger.
The batteries it is meant for (according to the product description) are ones for JVC cam-corders with 7.4V.