I hadn't heard about a third memo, but with the period of time that had elapsed between memos, it certainly looks like Mueller's team was manipulating the system to suit their needs, and Rosenstein capitulated each time. Every time they found someone or something new they wanted to investigate, they'd go running to Rosenstein, and he'd provide the means for them to do so. Rosenstein was a willing participant.
21 Apr: ConservativeTreehouse: Second Scope Memo – Rosenstein Authorizes Mueller to Target Michael Flynn Jr…
by sundance
The original authorization for the appointment of Special Counsel Robert Mueller was May 17th, 2017. However, the recently released Weissmann report shows there were two additional scope memos authorizing specific targeting of the Mueller probe. The first scope memo was August 2nd, 2017, OUTLINED HERE, and is an important part of the puzzle that helps explain the corrupt original purpose of the special counsel.
The second scope memo was issued by Rod Rosenstein to Robert Mueller on October 20th, 2017...
https://theconservativetreehouse.com/2019/04/21/second-scope-memo-rosenstein-authorizes-mueller-to-target-michael-flynn-jr/