What is this menu bar icon with three connected dots and a down arrow?
LibreOffice update icon.

Googling the image led me to MacRumours Forums


  It is the LibreOffice update dialog box that triggers this. Clicking
  that icon opens the LibreOffice update dialog box, but it does not put
  it in front of other applications. That is what is leading to
  everyone's confusion about this icon.
  
  To verify, I closed the update dialog, put LibreOffice in front of my
  other windows, and clicked the icon. The update dialog appears again.