During the year, one says both Tikkun Rachel and Tikkun Leah from midnight. During the Three Weeks, one says only Tikkun Rachel from midday. But at midnight during the Three Weeks, does one still say both Tikkun Rachel and Tikkun Leah (or only Tikkun Rachel, like from midday)?
Yes, based on The Ari Zal and The RaShaSh you say both tikunnim except on Rosh Chodesh where you recite only Tikkun Leah and Shabbat where you say neither. On Tisha b'Av itself at Chatzos HaLayla we only say Tikkun Rachel.
This is how almost everyone (if not everyone) who merits to say Tikkun Chatzos is noheg.
.כל המתאבל על ירושלים, זוכה ורואה בשמחתה