As long as the envelope is not completed or voided then the envelope can be "corrected" to add Recipients. In your example you mentioned that 3 people signed but did not mention whether this completed the envelope or if there were more Recipients expected to sign?

Here is a DocuSign article on correcting an envelope....

https://support.docusign.com/en/guides/ndse-user-guide-correct-documents

Hi there.

I’m having the same problem, but the link is broken…

Could you steer me in the right direction.

@JohanSR 

Try this:

https://support.docusign.com/s/document-item?language=en_US&bundleId=xrt1643059998084&topicId=xlr1662755460389.html&_LANG=enus

or you can go to DocuSign - support - and type in “correct an envelope”

good luck

mr1