import pandas as pd 

# create a sample dataframe 
df = pd.DataFrame({'A': [1,2,3,4,5,6,7,8,9,10], 'B': ['a','b','c','d','e','f','g','h','i','j']}) 
    
# select every 3rd row using groupby 
df_filtered = df.groupby(df.index // 3).first() 
    
print(df_filtered)