It's not a glitch - there are most likely no 3-person rooms left in the area you're looking. When you book online, it makes you book a room for your exact occupancy, so when you put 3, it only gives you rooms with 3 beds - it won't show you any 4-person rooms. All the 3-person rooms may be booked, or there may not be many 3-person rooms in that area. What ship and floor/area are you looking to book? I would recommend checking out the pdf deck plans to see how many 3-person rooms are actually in that area.
With that being said, we just stayed in a Family Harbor interior on the Panorama last week and I can't say enough good things about it. The room and lounge were great, and I'm happy to answer any questions you may have.