Help me understand this predicate logic question

[mattst88]

Homework Statement

Write a sentence of predicate logic which intuitively holds in a model iff the model has (respectively)
(a) exactly three distinct elements
(b) at most three distinct elements

I've completed the rest of this homework assignment, but I don't understand this question. Otherwise, the section consists of proofs using predicate logic.

Could anyone shed some light on what this question means?

The book (and the question) is available online, here

 

[mattst88]

OK... well. Don't bother now.