d = {"genre": ["A", "C", "A", "A", "A", "B", "B", "C", "D", "E", "F"]}
df = pd.DataFrame(d)
df
top3 = df["genre"].value_counts().nlargest(3).index
top3
df_update = df.where(df["genre"].isin(top3), other = "others")
df_update