what's wrong with the third Card?

My cards is R9 390 ,and everything looks ok.

But the third card's speed is too slowly :

Speed.Dev.#1.....:   178.5 kH/s (6.48ms)
Speed.Dev.#2.....:   178.1 kH/s (6.55ms)
Speed.Dev.#3.....:    50346 H/s (11.30ms)
Speed.Dev.#*.....:   406.9 kH/s


I've looked up wiki and found nothing, what should i do?

It's strange!!