Trinity/Talk

"The three parts of the Holy Trinity are widely held to be coeternal, of the same substance, and yet inexplicably different."

I have seen this explanation many times before, and found it incomprehensible each time. Can this be rewritten? By definition, if there things are co-existent, co-eternal, and of the precisely the same substance, then they are different. If these three things are different, then they are not precisely the same! Am I missing something here, or is this a a re-working of Tertullian's claim that one must believe this to be true because it is incomprehensible? If so, then I guess it should be left as is. Understand that I am not criticising the belief in the Trinity - I am merely trying to find out what just what trinitarians believe in, and I am unable to parse the claims about it. This explanation contains no recognizable content.