I had a FP3 which also from one moment the other went OFF and i was not able to turn it ON again.
The Battery was charged and the USB port cleaned - dissassembled all and cleaned all connections - but nothing helped.
As i had a lot of Fotos and Data on the phone which i did not have backedup yet - i wanted to find a solution.
So i took apart the FP3 and studied the marks on the chips of the mainboard and found by searching the ICs in the web that a defective Qualcom Power Managment Chip PMI632 sometimes causes these issues in other mobile phones.
I ordered some of these PMI632 chips from aliexpress (10 pcs for 13 USD) and found another FP3 with defect mainboard on ebay for testing without risking my own data.
And indeed it really worked!
Now my FP3 and the one i bought on ebay both are working fine again.
I just replaced the PMI632 with a temp controllable hot air gun at 380 °C and preheating the motherboard to 160 °C on a heatplate for several minutes. Important is to use a good soldering paste (flux paste) during the process. Before doing this i watched a couple of youtube videos where they replaced the PMI632 or similar BGA chips. The ordered PMI632 have allready new solderballs attached so reballing was not needed.
The PMI632 sits under the upper shield next to the battery connector on the side of the PCB with the display connector. Be careful removing the cap of this shielding case and do not bend the thin metal sheet - because underneath are attached heatpad.
Some people also reported that it is possible to switch ON a dead FP3 if you slightly bend it.
Because these Power Management ICs get hot during charging, sometimes indiviual pins get a cold solder joints and so bending the mainboard in the right direction can temporarily reconnect the lose solder joint and thus fix the problem.
In that case it should not be necessary to replace the PMI632.
Instead it should help if you just preheat the mainboard to 160 °C, apply flux paste to the IC and then use the hot air gun so that the ball pins of that chip resolder.