Haven't done three 600's but I do have two 1000's with a 6" venting system. I have fans everywhere. A/C, veg room and mom room plus 740 and 430cfm carbon scrubbers. A little noisy. Insulated duct reduces the noise a bit. I like to use more rather than less with fan size, then I use a 5 amp ceiling fan controller to adjust speed if I wish. 3 600's should be fine with 6" ducts but I'm not sure about the 4". Regardless, even the 4" will help some.