The Whatcom County Association of Realtors is fortunate to have an excellent attorney as our Government Affairs director. I am currrently President of the Association and have had long discussions with him on this very topic. The 3 unrelated persons ordinance is an old one on the books that just has not been taken off. It was originally enacted long ago so that who lived in neighborhoods could be controlled. In fact, there was a request many years ago to an attorney for a suite to be filed against a gay couple with an additional resident. The attorney wisely declined to pursue it. Enforcement of this brings up huge constitutional issues. How does the government say who gets to live together?
That being said - this is a very touchy subject in the York and Sehome neighborhoods as home owners there don't like a groups of college students living next door. Students often have parties and the properties and home upkeep is less than a typical home owners. In my view, there is no way to constitutionally enforce this rule but there is no pressing issue for the city council to delete it from the books.
So for investment purposes - I see no legal reason that it cannot be rented out to 3 or more unrelated persons. If you see an opportunity for investment, go for it.