Bald eagles do mate for life but will pair up anew if the mate dies or is killed. Both sexes are ferocious defenders of their nest; the males may be more territorial about the general area of the site the pair choose to build a nest in.
Not sure why this pair of males have tolerated each other. I suppose it's possible they are siblings or perhaps one just showed enough deference to the other to be tolerated at a time it came in handy, perhaps having mated with the female on the sly sometime, and then ending up helping fight off other raptors attacking the nest. The original female disappeared after one such fight and has been presumed dead or at least missing. Wildlife refuge managers looked for her but found no sign of feathers or a carcass.
In any case it was in 2016 that the researchers were first able to capture enough video footage to document that both males mated with the original female and all three took care of the fledglings.
The Upper Mississippi River National Wildlife and Fish Refuge is assisted by SUMMR (Stewards of the Upper Mississippi River Refuge) and they are who run the refuge's webcams.
The writeup on SUMMR's webcam site --the cam doesn't autoplay, so just scroll down to the Past History section-- about the full history of what is now an apparently enduring threesome was fairly hilarious in noting that at first the original male of the trio "didn't seem to know what to do" in his first known season of having mated with the original female, i.e. had no apparent sense he had acquired a species-preserving responsibility to help manage the consequences of mating behavior. Those hatchlings did not survive with only the female paying enough attention. He apparently got better at this after awhile, even if it took another male to help him learn how to man up for his share of parental duties.