Endonucleolytic cleavage within the 3'-External Transcribed Spacer (ETS) of a tricistronic rRNA transcript that contains the Small Subunit (SSU) rRNA, the 5.8S rRNA, and the Large Subunit (LSU) rRNA in that order from 5' to 3' along the primary transcript. In S. cerevisiae, endonucleolytic cleavage within the 3'-ETS of the pre-RNA, which may occur cotranscriptionally, is the first step in rRNA processing, and initiates a cascade of subsequent processing and modification events.