import pandas as pd
file1='C:/Users/Administrator/Desktop/00/1.xlsx'
file2='C:/Users/Administrator/Desktop/00/3.xlsx'
file3='C:/Users/Administrator/Desktop/00/21.xlsx'
file=[file1,file2,file3]
li=[]
for i in file:
  li.append(pd.read_excel(i))
writer = pd.ExcelWriter('C:/Users/Administrator/Desktop/00/output.xlsx')
pd.concat(li).to_excel(writer,'Sheet1',index=False)
 
writer.save()