to go part time?

[newlifebegins]

thinking to cut to 3 days a week. is it better to request it by email/ letter? do I need to say reasons?

will my part time contract ( permanent) - be as secure as full time?

I think this depends a bit on the culture at your school / your relationship with the HT.
Personally I felt it only right to go and chat to the HT before putting in the formal request, but I know not all HTs are as approachable as others.

Yes, your contract will just continue, with only the amount to work changing.

only the amount to work changing
reducing slightly...

There is a formal procedure to requesting part time working, look on the TES forums, they have an exemplar letter.

Are you primary or secondary? Be aware that at secondary working 3 days a week may not mean getting 2 days off. I work 0.6 but have to be in 5 days a week.

Yes, I did 0.6 as a 3 day job share, it was great we really did share and was very manageable.i could work for one of my days off and have a day off.
I was changed to mornings only, all week. the work load just doing English and maths with a year 5/6 class didn't feel like part time anymore, and lost the luxury of spare time. Hence I left and am now a flexible supply teacher.