These balancers will “see” the 3 cells as one cell so yes they will work in exactly the same manner. The only job that the balancers do is balance, but they do it better than the BMS can as they move more current actively whereas the BMS can only blow off of the excess charge of the “too high charged” cells as heat. The BMS also monitors for excess heat, excess high voltage, and too low voltage by shutting off that battery. Do be sure to get all the wires to the correct locations , check and double check, it just takes a fraction of a second to fry the delicate components of the BMS and balancers. Both devices do their job independently of each other. Do be sure to check the voltages on each pin of the connectors at each connector before plugging each connector to the BMS and the Balancer.
The one advantage of the Overkill package that you bought is the support that they provide. Very good instructions along with good support of the product.
You could have bought the same item from China but with zip squat nada support and documentation written by persons who speak and write pigeon english. So you made the right choice in buying good support along with a highly regarded BMS.
From what I have read the QMBBS balancers are the top of the line, but I did not want to put that much money in unproven cells so I opted for what I perceive as the “next best”
Please do note that all of my experience is with far smaller lithium battery packs, with only a small amount of power for portable applications. I have sucessfully repowered devices with lithium cells using no BMS on discharge but using a charger/balancer/discharger/tester to recharge. Absolutely no fails in about 5 years doing this. I have a fascination with model airplanes that can break 100 m.p.h. using only a small battery pack. That is where i started doing lithium.
The person whose knowledge of large scale LFP cells that i respect the most is on this forum as
@Steve_S , you would do well to read some of his posts, he has a quite large set of lithium cells to run his homestead.
Would love to join you on a cruise in blue water but I am in California for medical care which I cannot find in Alaska, my home ground......Now that you have identified which cat that you cruise on I will look it up on the internet, My Trimaran was more of a backpackers rocketship on the water, Wicked fast with minimal accommodations, guess that i am a speed sailing junkie, had a Hobie cat, wicked fast...