i just started menstruation and my first period was 5, the second was 7 and third one only one day

There appears to be no cause for concern. It can take several months for your system to settle down. You could find that your periods last a varying number of days, and your cycle lengths my also vary. So it may be a while before you can ascertain both the day your period will start, or how long it will last.

I hope this helps.