The "three board" limit applies to editable boards. You can create a "Master Archive" board. When a project finishes, copy the final data into the Archive board (turning it into a static image), delete the original, and free up a slot.
You can be invited to an unlimited number of boards created by others (even paying users). If you are a contractor or a student, you can participate in 50 projects simultaneously as long as you aren't the owner of the board. miro board free
Keep in mind that Miro offers paid plans with additional features and scalability for larger teams and enterprises. If you outgrow the free plan, you can upgrade to a paid plan to access more features and capabilities. The "three board" limit applies to editable boards