Hi there.
I'm currently training to do the 10km for Sport Relief in March, just to help you put this into some better context the usual winners of Sport Relief 10km finish in around 35-40mins, these are the fittest people on the day, so by the formula you have heard of "anything above 12mins to run 3km is poor" if you add a little bit more 3km is essentially a third of a 10k and so 3(km) x 12(mins) = 36(mins), so by this logic you're going by or something has told you to go by it is that of potentially one of the fittest people on the day of thousands running for Sport Relief, seems like a pretty harsh grade to go by, do you catch my drift?
I'm running 3 times a week now and mixing up my on/off, so I run for a set amount of time and then rest(fastwalk/light jog) for a set amount of time and repeat for 25 minutes. In 25 minutes I usually do around 3.40km, this is about a third of what I will need to do to complete my 10km run for Sport Relief.
The time it takes is not bothering me, it is that I am fit enough to make sure I do the full 10km in a steady pace. So I'm aiming for an hour, I would like to do it in around an hour and I'd be pretty happy with that.
I think whatever you should be aiming for depends on what you are trying to accomplish, I am trying to increase my general fitness and stamina, sure timing myself helps me keep track but if I were you I wouldn't put so much emphasis on how quick you are.
If you want a straight answer, seeing as your a bit out of the exercise thing (I was too until recently) I'd recommend doing what I do, 3 runs a week and change how you run, for example run fast pace for 1 minute then rest(quickwalk/light jog) for 1 and a half minute, or run very hard for 30 secs rest for 2 minutes, or run fast pace for 2 minutes and rest for 3 minutes..
If you keep mixing it up you'll find your fitness increase quickly because it's giving your lungs and heart a really good workout, getting that heart rate up and then bringing it down really works it well and you'll see quick improvements like I have!
DON'T be disheartened by other peoples fitness, muscles or knowledge; use it all to inspire you into getting to exactly where you want to be too!
Hope this helped bud!