column number instead of column name

how can i make use of the column number instead of the column name when making the select statement?

for instance:
"SELECT mytable.col[1], mytable.col[2], mytable.col[3], mytable.userid from mytable"

i have several tables and i need to extract the first three columns and a known column. the first three columns for the different tables vary, but all the tables have the userid column.

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