Adding a matching value from 3 different DataFrames, not the entire column Python

Question

I have three different DateFrames (df2019, df2020, and df2021) and the all have the same columns(here are a few) with some overlapping 'BrandID':

    BrandID  StockedOutDays  Profit    SalesQuantity
243 01-02760     120        516452.76     64476
138 01-01737     96         603900.0      80520
166 01-02018     125        306796.8      52896
141 01-01770     109        297258.6      39372
965 02-35464     128        214039.2      24240
385 01-03857     92         326255.16     30954
242 01-02757     73         393866.4      67908

What I'm trying to do is add the value from one column for a specific BrandID from each of the 3 DataFrame's. In my specific case, I'd like to add the value of 'Sales Quantity' for 'BrandID' = 01-02757 from df2019, df2020 and df2021 and get a line I can run to see a single number.

I've searched around and tried a bunch of different things, but am stuck. Please help, thank you!

EDIT *** I'm looking for something like this I think, I just don't know how to sum them all together:

df2021.set_index('BrandID',inplace=True)
df2020.set_index('BrandID',inplace=True)
df2019.set_index('BrandID',inplace=True)

df2021.loc['01-02757']['SalesQuantity']+df2020.loc['01-02757']['SalesQuantity']+
df2019.loc['01-02757']['SalesQuantity']

Answer 1

import pandas as pd
df2019 = pd.DataFrame([{"BrandID":"01-02760", "StockedOutDays":120, "Profit":516452.76, "SalesQuantity":64476},
          {"BrandID":"01-01737", "StockedOutDays":96, "Profit":603900.0, "SalesQuantity":80520}])

df2020 = pd.DataFrame([{"BrandID":"01-02760", "StockedOutDays":123, "Profit":76481.76, "SalesQuantity":2457},
          {"BrandID":"01-01737", "StockedOutDays":27, "Profit":203014.0, "SalesQuantity":15648}])


df2019["year"] = 2019
df2020["year"] = 2020

df = pd.DataFrame.append(df2019, df2020)
df_sum = df.groupby("BrandID").agg("sum").drop("year",axis=1)

print(df)
print(df_sum)

df:
    BrandID  StockedOutDays     Profit  SalesQuantity  year
0  01-02760             120  516452.76          64476  2019
1  01-01737              96  603900.00          80520  2019
0  01-02760             123   76481.76           2457  2020
1  01-01737              27  203014.00          15648  2020
df_sum:
          StockedOutDays     Profit  SalesQuantity
BrandID                                           
01-01737             123  806914.00          96168
01-02760             243  592934.52          66933