Dude - no. Focus on quality of movement. I don't even want to hear why you'd want to condense your workout to 12 minutes.
For starters, I'm pretty sure you're not going to be able to do a full 20 seconds of pull-ups if you're doing them as fast as you can like tabata suggests, unless your numbers for pull ups are already above 15. So you're going to have shitty quality especially as you start to gas out. Same goes for push ups.
Sit-ups, I guess here you can knock yourself out. In my opinion sit ups are kind rather mindless and your time is better spent doing anything else (planks, leg lifts, etc) but they would lend themselves better to a tabata scheme.
If time is of the essence, I would do one or two tough sets of pull-ups and push-ups (weighted) to get the most out of your workout - Body by Science style.
I used to seriously post here, now I prefer to troll.