It is certainly possible that they were infected by one or more of the three previously infected ones. It is also possible that others were very recently infected but did not have sufficient time to produce enough virus particles to register positive before sailing. It would take some time after infection for most people to test positive.
More to the point - they make a big deal out the impact of neutralizing antibodies on the ability of the three not to develop another infection. Considering how new the virus is and that it has had little time to mutate, many experts would be surprised if the three did catch the virus again. That is why they call them neutralizing antibodies. It is also commonly known as "immunity". So it comes as no surprise, really.
There are countless stories of people avoiding reinfection by viruses and bacteria due to previous exposure to them. This is why vaccines often work so well. But, considering all the surprises this virus has caused, it is good to see that it is not a super virus, and it is subject to neutralizing antibodies of a prior infection.