I have had people lose a duck when they get a breeding pair and it is hard on a duck to lose a close companion as they are very social and form strong bonds. I sell a minimum of 4 ducklings together with the hope that none will be singled out.
The strategy of adding two is a good one so you will never have a lone duck again (unless you lose two at once) but it sounds like the two are bonded together and not willing to include a third duck. Your duck will attach to them out of loneliness but they may be a bit reluctant at first. I would watch them and let them get to know each other. At least your duck is not completely alone now but ideally it will be able to form a close bond with the companion ducks.