Our board needs to hold an executive session covering three different topic areas of the executive session statute. Do we post these as three separate agenda items, three separate meetings, or as one agenda item or one meeting?

You have some options, and could possibly do it as any of the things you listed. However I believe the easiest and most straightforward way to post is as one meeting, with one executive session covering three separate parts of the statute. This is true whether it is the only agenda item at the meeting, or if the executive session is only a portion of a much longer public meeting.
The posting would look like this:

EXECUTIVE SESSION- The board will meet in executive session under ORS 192.660(2)(a)To consider the employment of a public officer, employee, staff member or individual agent; under ORS 192.660(2)(d) To conduct deliberations with persons designated by the governing body to carry on labor negotiations and under ORS 192.660(2)(h) To consult with counsel concerning the legal rights and duties of a public body with regard to current litigation or litigation likely to be filed.

The board chair should read the posting statement above out loud in public session before the board begins executive session. Then he/she can read the press their instructions after they get into executive session.

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