can any one give me a first-order sentence that says there exist exactly three elements. Please explain it to me.
How about this (assuming you mean that there exist exactly three distinct elements):
This says that there are three elements that are mutually unequal, and if you consider any element at all, it must be equal to one of those three elements. Does that do the trick?
Thank you very much about that. I found this in a book but without example about the first order sentence that says there are three elements.
So , as I understood , I just write a sentence that has three elements which are distinct. is that correct