Output one fixed width file using three different data streams

In my workflow I have three separate data streams that I need to be output as one fixed width file.

Stream 1: The "header record" is currently 8 columns with varying column lengths (all data types are strings)

Stream 2: The "detail record" is currently 113 columns with varying column lengths (all data types are strings)

Stream 3: The "trailer record" is currently 111 columns with varying column lengths (all data types are strings)

How do I go about combining the three data streams and outputting them as one fixed width file while preserving the individual column lengths required for the header, detail, and trailer records?

Thanks!

Courtney