Slz1900 is right about the sounds.
However I'd normally consider an S3 to be a murmur. There's only no murmur if it's S1/2 + nil. I'm not a cardiologist so maybe I'm wrong, but if there's a reputable source saying S3/4 aren't murmurs I'd be keen to see it.
Apart from that, the vignettes will differ substantially. S3 is normally seen in someone with heart failure with reduced ejection fraction/systolic dysfunction. The heart will be dilated on CXR and apex beat displaced laterally. Yes, you can get S3 in pregnancy and in young athletes, but for the sake of the USMLE, heart failure is lightyears more common.
MS is seen in adults with history of rheumatic fever. 99% of MS is due to prior RF. RF causes MS acutely, but it becomes MS over time. The <1% of MS is Libman-Sacks endocarditis seen in SLE (increased if + lupus anticoagulant). So for MS they'll mention an immigrant most often, or someone who had MR as a kid after an infection. MS can also appear acutely during pregnancy (as does S3), but MS will present as acute dyspnea in pregnancy due to pulmonary oedema from sudden increase in preload; the S3 in preggers is most often incidental.