Yes, they're fine. The middle sentence has a slightly different nuance in meaning, however. Let's not say harsh words to each other.—This asks for agreement to not use harsh words to each other. Let's not use harsh words for each other.—This asks for agreement to not use harsh words AS DESCRIPTORS OF each other (perhaps to other people). Let's not exchange harsh words with each other.—This asks for agreement to not use harsh words to each other.

Let's not use harsh words with each other.