This relies on you being a trusted and valued worker for the firm who allows you to work 3 days. So work hard at your job and the management will likely be okay with you asking.
That's what I did for a couple of startups.
Having a work-day or two available for work-time meetings with customers and lawyers / suppliers makes a large difference. People treat you more seriously when you are able to make a daytime meeting.
Careful calendar managament can give the impression of a full fledged business.
Until of course you need to support customers, and suddenly they need support at all sort of inconvenient times. Nothings perfect!!
But yeah, 3 days a week is immensely better than working 5 days a week for someone else, then trying to get something done on the weekends.