The preset files are really more for when you have a load of really complex options, and they're applied on a per-codec basis. In your case, I don't think they'd shorten your command line by any useful amount.
I think the easiest way to achieve what you want would be a bash script.
This takes advantage of positional parameters. Save it as 'my_avconv' somewhere in your $PATH (like ~/bin) and set it as executable, and then use it like this:
avconv -i "$1" -s 960x540 -vcodec libx264 -acodec libmp3lame -b:v 3000k -b:a 192k "$2"
As usual, quote file names with spaces.
my_avconv input.file output.file