2. Oh ARSE! I have spilt my pint.
3. Your mouth smells of arse.
Until the Victorian period, "ass" had no profane meaning and simply referred to what we now call the donkey. Because of the increasingly non-rhotic nature of standard British English, "arse" was often rendered "ass". The age of Victorian propriety resulted in the rechristening of the horse-like animal, changing the name to "donkey" to avoid any improper inferences. This usage was also adopted in America, which is why the word "arse" is not usually used in the United States. Some people in Britain have adopted the American version in writing. Although before World War I they were similar, the British pronunciations of "ass" (IPA /æs/) and "arse" (IPA /α:s/) are now quite different.
Sorry, but "ass" just sounds more pleasantly vulgar.