1 I've already aired the room this morning
--------一般感觉,说这句话的时候,时间仍然是morning time.
2 I aired the room this morning / in the morning
-------一般感觉,说话的时候已经不是 morning time了,所以用“一般过去时”.
3 I have been airing the room since 7 o'clock this morning
--------因为since 7 o'clock this morning的参与,你必须用“现在完成进行时”,要知道,在你的语境裏面,作为动词,air是瞬间性动词,不是延续性动词.