There's no way to do that natively with Google Calendar. There might be a way to do it with ICAL, but you'd need a client that could create the events with the right setup or to build it manually by hand.
Since every three workdays is a repeating cycle...
- Monday, Thursday
- Tuesday, Friday
- Wednesday
- Monday, Thursday
- ...
In week 4 the pattern starts again.
One not-too-painful workaround would be to create three separate events, each with the same information.
Here's how you can do that:
- In week 1, create the event on Monday. Set it to repeat Weekly, every 3 weeks, on Monday and Thursday
- In week 2, create the event on Tuesday. Copy the information from the event in week 1. Set it to repeat Weekly, every 3 weeks, on Tuesday and Friday
- In week 3, create the event on Wednesday. Copy the information from the event in week 1. Set it to repeat Weekly, every 3 weeks, on Wednesday
It's not as simple as a single event, but at least it's only three to manage.