If I was actually giving advice, I would suggest - Do more than you need for a test - then you'll have the confidence of knowing that you're going to pass and can go for a 'best time'.
If you're just starting training on the treadmill, go for 6k at 12kph.
Once you can achieve this regularly do 3k at 12kph, then without a break, increase the speed for the second 3k to 13kph.
Again, once comfortable with this, increase the second 3k to 14kph and so on, through 15, 16, 17 and 18kph, or wherever you max out.
Once you find a max level for the second 3k, that you can do consistently, do the final 100 metres, or 0.1k as your best sprint.
Gradually increase the distance of this final sprint to 2-300 metres.
Occasionally, once every week or two, test yourself at the official 2.4k at 12kph, rest for the minute and do 2.4k starting at 15kph gradually increasing the speed to achieve a best effort.
Once you have achieved the first major goal of 3k at 12kph followed by 3k at 15kph, your confidence should be sky-high. You'll know you can pass the 3-miler and any extra gain is a bonus.
I'm not EVER going to PRMC but if I was, that is how I would train.