df2.columns = stocks['Ticker'][:3]

[:3] is just use first 3
[5::] skip first 5



                price price  price 
2021-01-11    131.90    15.00    179.07
2021-01-12    128.09    15.74    182.65

to

Ticker    A    AAL    AAP
2021-01-11    131.90    15.00    179.07
2021-01-12    128.09    15.74    182.65