Little help with this challenge please

[doctrinederp 6]

"use 3 transmitters"
I've apparently used 2/3, although I don't have the plan unlocked and don't recall ever looting one from a corpse.
I figured maybe it meant the signal detector or comms station, but i've used both of those several times since, and i'm still on 2/3
https://gamerdvr.com/gamer/derpatologist/screenshot/14419274#

[wyvern01 12]

It only counts actual transmitters placed, but for some reason it randomly adds to your count.  There is no know way to trigger the addition, and if you wait a few days hoping to fulfill it randomly then the game will actually reset the number to 0 at some point. 

[Matthew Aiken 132]

Take it as a free bonus tbh and if you need more search for them dig spots especially on boosted games I use fiske and fjellkanten as I know every spot off by heart wherever I spawn I start searching them haha