As far as the first two sentences go, they mean the same things. However, a couple points of usage:
if you were having guests.
If is not necessary in this sentence. The word like here means similar, so it is not a conditional, but rather a comparison. The original sentence uses if because of as—how should you dress? As someone expecting to have guests = as if you were to have guests. Also, the original sentence might be more succinctly found as "Dress as if you were having guests".
She kissed him goodbye, like
The is not necessary, like the original sentence.
The third example, however, is different, as it uses a different meaning of as
Sweet like she is, she doesn't pay his bills.
This construction is not found among native English speakers, who always use the word as. The reason is this: like expresses similarity, but in this context, as expresses quantity; in other words, the sentence is saying that she is very sweet. The altered sentence, however, is meaningless.