Try this:
if(
isNull({[~]Tag}),
"0:00:00",
stringFormat(
"%02d:%02d:%02d",
toInt(floor({[~]Tag}/3600)),
toInt(floor({[~]Tag}%3600/60)),
toInt(floor({[~]Tag}%60))
)
)
If your float value is too large the first value will have more than 2 digits.