Why isn't this working, and can someone fix it?

[blacktraffic]

I think you need to re-order the arguments, so hash.txt needs to come earlier on, --show doesn't apply and -a3 requires a mask to be specified. The following works for me:

hashcat64.exe -O -w3 -m0 --username hash.txt.txt -a3 ?a?a?a?a -o cracked.txt

where hash.txt.txt is:
someone:5f4dcc3b5aa765d61d8327deb882cf99 [ test hash ]