Fortunately this doesn't need to be done in one long continuous 3 hours stint and can be broken up into smaller sessions.
There's three ways of getting this ...
1. Find someone who either needs it or is willing to give 3 hours, climb up a tall building away from random enemy interruptions and leave the game idle for 3 hours. Make sure to disable the power saving options for the controller and console in PS4 settings menu beforehand.
2. Use the built in matchmaking to find a partner. The problem with this is the host will most likely kick you for not helping so you will have to find another partner. I did it this way and i played around 7 games total before i got the full 3 hours.
Another problem with matchmaking is every time it fails to find a partner it kicks you back to the main menu requiring several button presses to get back to searching for one. Why they didn't put a retry option in is anyone's guess.
Keep trying is the best advice, you should get a partner in 5-10 tries.
3. If you've only just started playing, set the CO-OP privacy option to "open to all". This will allow you to get it while playing naturally by accepting requests to join your game. You should have the trpohy by the end this way through the combined amount of time spend allowing randos in.
This and the "Gnarly Wave" trophy are the 2 most annoying in the game IMHO.