2x120mm 12V fans blowing air up under the base of the bottom one's.
airflow created by the fans of the U3 to cool the base on the middle and top ones.
They're in my garage, so temperature is between 5<25C (and changes gradually) over the year.
I would think the cooling depends ambient and how hard you push them. Look at batch 1's they thought they were..... and they had lots of issues.
But you did have batch 2's. 25 c does not sound to bad. If you might use the IR temperature gun if you have one. I thought the metal bottom could get a bit hot when running, but I could be wrong.