I am currently running them open air. I tried them in a rig but the 2nd card is too close to the 3rd card and gets throttled to 700 kh/s, even with max fan speed. This was without the lid, when I close the lid it throttles a lot and temperatures raise 5-8 C.
Are you running the same motherboard? Do they all physically fit in case?
Regarding the heat issue, I have the same thing now in a desktop case. In the 4u case I plan on putting a very high airflow (200CFM) fan directly on card 2&3.
Do you have any photos by any chance?