第3月曜日 3rd Monday or Monday in the third week 3 I'm not sure the actual meaning. 1. 3rd Monday that some month may not have this day ( Monday that the date is the third ) for clearer explainantion in picture below 

3 refers to the Monday that comes for the third time in the given month. All months have 3 because all months have at least 21 days. Likewise, there is no month that has 6. Note that 3 in Japanese never refers to the third day of a month in the first place.

Also note that your definition #2, "Monday that is in the third week", is probably inaccurate. For example, The 3 of September 2020 is the 21th (see the calendar below), but I think many people think "Monday that is in the third week" refers to the 14th.

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