Postinor 2 tablet contains a synthetic luteal hormone-like active substance,levonorgestrel.
Postinor 2 tablet prevents about 85% of expected pregnancies when the first tablet is taken within 72 hour's (3days) of unprotected sex. Postinor 2 tablet Will not prevent pregnancy in every instance, but the tablet are more effective the sooner after unprotected sex they are taken(it is better to take the first tablet within one day rather then delay until the third day). You need to take the second tablet 12 hours (but no more than 16 hours) after the first.
Postinor 2 tablet is thought to work (1) by stopping your ovaries from releasing an egg.(2) by preventing sperm from fertilising an egg you may have already released.

for more info.....
http://www.humblemedicals.com/2018/11/what-postinor-2-is-and-what-is-used-for.html

Can my girl take postinor 2. ... 3times in a week? Pls reply

You should only use Postinor 2 tablet in emergencies and not as a regular method of contraception. If Postinor 2 tablet is used more than once in a menstrual cycle it is more likely to upset "her" menstrual cycle (period).