yeah this has been bugging me for like three days now. finished up my build last weekend, everything powered on first try which honestly shocked me, but motor 3 just does this weird twitch thing when i arm it. the other three spin up clean when i give it a tiny bit of throttle, motor 3 just kind of stutters and vibrates in place. sometimes it'll catch and spin normally, sometimes it just sits there buzzing.

i reflowed all the motor pads on that ESC corner, checked continuity on the motor wires, even swapped the motor with one from another arm and the problem followed the motor which made me think it was the motor itself. but then i put a different motor on that same ESC pad and THAT motor also twitched so now im totally lost about whether its the ESC or the motor or something in betaflight

running a 45A 4in1, motors are 2306 1700kv on 4S. betaflight shows all four motors in the motor tab and they all respond but 3 is just wrong. havent maiden it obviously.

check your motor output in betaflight ? go to the motor tab and spin each one individually. if motor 3 stutters even with a clean signal from the FC then its almost certainly either a cold joint on the ESC phase wires or youve got a damaged FET on that corner. the fact that two different motors showed the problem on that same pad points pretty strongly at the ESC.

also worth checking if there's any physical damage on the underside of the ESC around motor output 3. sometimes you get a hairline crack in a trace that passes visual inspection completely but breaks the circuit enough to cause dropouts under load. had this exact thing happen on a Mamba stack after a bad crash, looked totally fine but one FET was basically half dead.

desync or bad motor ? those are basically always the two answers with this kind of thing. did you set your ESC protocols right in betaflight? if its on oneshot or multishot instead of dshot you can get some really weird behavior that looks mechanical but isnt. i know it sounds dumb but ive wasted an afternoon assuming hardware fault when it was just dshot not being enabled on the right UART. just worth double checking before you go reheating joints again