code: #%% 
  
  import pandas as pd
  
  #%%
  
  df1 = pd.read_csv('df1.csv', index_col=0)
  # %%
  df2 = pd.read_csv('df2.csv', index_col=0)
  # %%
  df3 = pd.read_csv('df3.csv', index_col=0)
  # %%
  df1.merge(df2, on='proj_id').merge(df3, on='doc_id')
  # %%
  df1.merge(df2, on='proj_id', how='left').merge(df3, on='doc_id', how='left')
  # %%
  
apis: pandas.read_csv