I work 3.5 days a week. I dropped when I had kids and never gone back up.
While I love it and it makes putting up with the job much easier, sometimes I'm aware that my workload is the same put I'm paid less.
It undoubtedly affects your promotion prospects and your pension.
I could work more hours now the kids are late primary but there's still a lot of appointments, plays, drop offs to things that doing less hours provides the flexibility to accommodate.
We could have more money but we've nearly paid off the mortgage on our small house and have reasonable savings. I wouldn't do it without something behind me, especially not single.
If you can manage on three days salary, why don't you have decent savings? Have your circumstances changed recently?
Would your employer consider the nine day fortnight or compressed hours?
If you live alone, also consider that you may find it lonely. People aren't always available on random week days.
It is lovely to have a shorter week. I'm prepared to make the sacrifices that come with it but in your circumstances, I wouldn't drop as low as three. Four days at a push would be a better compromise.