As the Stage 3 murmur can be caused by a no. of different reasons including TVD, seeing a cardiologist as already stated would be the best, most knowledgable vet to work with. A cardio has many sized stethescopes and a 2d color echo Doppler.

I personally feel, most cardiologists will recommend a Doppler immediately after auscultation. Try to find a clinic near you or a little further, it's worth the ride for a proper diagnosis. I hope it's something minor and you'll be able to let out at sigh of relief.