Work 3 days per week for someone else.

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!!