I just took and passed my NSCA-CPT a while back, and it was a hard test. I've lived in the medical community all my life, my mother was a nurse for 25 years, and I worked as a Biomedical Engineer for 2 1/2 years. I have a wealth of knowledge of anatomy and physiology as it was a hobby due to all my mother's nursing books lying around, plus my obvious involvement in weight lifting/training. I developed an interest in bodybuilding and proper training in the marine corps in the late 80's. I have trained many people, helped many people get good results, and decided to get a certification and do it professionally. So, I tell you all that to say this:
The NCSA CPT is a difficult test, but it is within reach, don't let anyone tell you any different. You'll spend several hundred dollars for study materials if you expect to pass this exam.
The NSCA CSCS- you have to have at least a BS in a related field to even sit for this exam. Need I say more?
Haven't researched the ACSM, but I've heard that it's a good certification, and it'll probably be very difficult as well.
But, any certification worth having isn't easy to get. Hope this helps.