For this achievement, you are required to catch 3 Boss fish in one sitting without backing out and quitting to the main menu. With the following set up I managed to catch a variety of Boss fish, so if you are patient enough you should have no problem catching 3 in a single sitting. I do not believe that the rod, reel and line need to be exact (i'm guessing high spec gear that will allow you to cover some distance when casting along with high breaking strain line is the correct direction) but if you can afford to purchase it in game, then I would suggest it.
Rod: Haichou 13ft Extreme
Reel: Sonik Tournos 10000
Line: Korda 25 Supernatural
Rig: Korda IQ-D Rig 4 (Silt)
Bait: Korda Dumbell 18mm (x2 rods)
Bait: Large Double Sweetcorn (x1rod)
Once you have your gear set up and ready, load up 'Foundry Dock' with just the original setting will do and choose the middle north peg as your start location. I used this peg almost every time and got multiple boss fish here.
It is also important to understand that this is obviously a fishing game and that it relies heavily on luck due to the RNG aspect of the game, so everyone's experience will be different and therefor does not mean you will get any of the achievements straight away.