How to convert Seconds to HH:MM:SS using T-SQL

You want to multiply out to milliseconds as the fractional part is discarded.

SELECT DATEADD(ms, 121.25 * 1000, 0)

If you want it without the date portion you can use CONVERT, with style 114

SELECT CONVERT(varchar, DATEADD(ms, 121.25 * 1000, 0), 114)