> # The file is tab delimited (your time field having spaces shouldn't matter)
> cat tmp.txt
3       1a      2       b
2       2c      3       b
1       3b      1       b
> # The char between the quotes is a tab ("\t" works in ksh)
> sort -t" " -k3n tmp.txt
1       3b      1       b
3       1a      2       b
2       2c      3       b
>