Perhaps you need to read more about SQL. Your syntax is missing the comma separators and the WHERE clause. Beyond the manual, Lafler and Prairie have good books in the BBU series.
If all the datasets have the same variables, including bill1-bill3 in each, then an equijoin will probably not get you what you want. The last data set in (ds3) will provide the data values and the data values from ds1 and ds2 will be lost.
If the incoming datasets have a common variable called, say, "bill" and you want to do an equijoin and rename it to bill1, bill2, and bill3 on output, that can be done.
ds1.bill AS bill1,
ds2.bill AS bill2,
ds3.bill AS bill3
ds1, ds2, ds3