Turns out a good database is
really good at data munging. A solution for postgresql might look something like this:
CREATE VIEW my_table_improved AS
SELECT SUBSTRING(the_column, 0, 8) as col1,
SUBSTRING(the_column, 8, 8) as col2,
SUBSTRING(the_column, 16, 8) as col3
FROM my_table
After which you can query my_table_improved as a normal table. col1, col2, and col3 contain the data split out from the_column. In practice you'd probably also want to do some type conversion, e.g. if col1 is supposed to be an integer you can simply update the view to select `CAST(SUBSTRING(the_column, 0, 8) as INTEGER) as col1` instead. In production use you might find this to be slow at which point you will want to create indexes for your new columns. Something like this (adjusted to which queries you're running of course) should work:
CREATE INDEX ON my_table ((SUBSTRING(the_column, 0, 8)));
Of course this is a lot of work if you're unfamiliar with SQL, and above examples aren't quite complete yet for your use case, but it should get you an idea of how SQL is the exact right tool for the job here. which is somewhat the point of many commenters here: get yourself familiar with SQL and save yourself a metric tonne of work in the future.
An alternate approach might be to write a query that migrates the fixed-width format to a format where each entry is in their own column. The ease of this mostly depends on if applications depend on that column being in that format.
side note: above sql code is untested but should be roughly correct.