下列三句句子的意思有否分別？

(1) We always enjoy it when they stay with us.
Grammar is correct, but it is, to a certain degree, ambiguous in meaning as we don't exactly know what "it" refers to.
Rewrite:
We always enjoy staying with them.
We always enjoy their staying with us.
We always enjoy the time when they stay with us.

(2) We always enjoy it that they stay with us.
"that they stay with us" is a declarative content clause functioning as a complement of the noun before. Therefore, we cannot just use a pronoun "it" in the sentence.
Rewrite:
We always enjoy the fact that they stay with us.

(3) We always enjoy that they stay with us.
"enjoy" is not normally followed by that-clause.

同意你的說法，多謝。

In response to AhShing's answer on sentence one, "it" is not ambiguous. It is a dummy object, that is, the word "it" does not refer to anything in particular. It serves as the object of enjoy and this is all.
Sentence one is perfect.

基本上係無～
不過，第一句比較好表達個意思，作文最好用第一句！