What is the average US deposits for 2017-2019?
{"At 31 March": {"2019": "\u00a3m", "2018": "\u00a3m", "2017": "\u00a3m"}, "Cash at bank and in hand": {"2019": 495, "2018": 446, "2017": 469}, "Cash equivalents -- US deposits": {"2019": 3, "2018": 26, "2017": 32}, "Cash equivalents -- UK deposits": {"2019": 1132, "2018": 31, "2017": 1}, "Cash equivalents -- Other deposits": {"2019": 36, "2018": 25, "2017": 26}, "Cash equivalents -- Total cash equivalents": {"2019": 1171, "2018": 82, "2017": 59}, "Cash equivalents -- Total cash and cash equivalents": {"2019": 1666, "2018": 528, "2017": 528}, "Cash equivalents -- Bank overdrafts (note25)": {"2019": -72, "2018": -29, "2017": -17}, "Cash equivalents -- Cash and cash equivalents per the cash flow statement": {"2019": 1594, "2018": 499, "2017": 511}}
us_deposits_2019 = df["Cash equivalents -- US deposits"]["2019"]
us_deposits_2018 = df["Cash equivalents -- US deposits"]["2018"]
us_deposits_2017 = df["Cash equivalents -- US deposits"]["2017"]
answer = ( us_deposits_2019 + us_deposits_2018 + us_deposits_2017 ) / us_deposits_2019