I had a friend who went for an ultrasound and they couldnt see one of the chambers of the heart, so she just went back for another pretty soon after and the 4th chamber was visibal.

With babies thier heart only operates as 2 chambers because the job of the placenta is to carry oxygenated blood into the body rather than the lungs, and there are little holes that close over within minutes of the birth to make it a 4 chamber heart so that is how I guess it is possible to have only 3 chambers when a baby is in utero.

If I was you I would make sure that I got my Ob to explain everything so I understood and then demand another ultrasound ASAP with someone different.