1. I bought a little mask for my sister, Sue.
2. I bought my sister, Sue, a litle mask.
3. I bought my sister, Sue a little mask.
(Which one is right #2 or #3? From #1 we can get #2 or #3, right? Do I have to put a comma after 'Sue'?)
#s 1 and 2 are correct. #3 would also be correct if the comma were removed or if there were a second comma after "Sue."
Both sentences #2 and #3 are grammatically correct, but they have slightly different meanings.
Sentence #2: "I bought my sister, Sue, a little mask." In this sentence, the name "Sue" is set off by commas, indicating that it is an appositive, providing additional information about the noun "sister." The sentence implies that you bought a little mask for your sister, who happens to be named Sue.
Sentence #3: "I bought my sister Sue a little mask." In this sentence, there is no comma after "Sister," so "Sue" is not being set off as an appositive. The sentence states that you bought a little mask for your sister named Sue. This version emphasizes that she is your sister and her name is Sue.
As for sentence #1, it is incomplete and does not form a proper sentence. However, it serves as the base for constructing sentences #2 and #3.
So to answer your question, it depends on the meaning you wish to convey. If you want to emphasize that your sister's name is Sue, then use sentence #3. If you want to provide additional information about your sister having the name Sue, then use sentence #2. Both are correct, but they have slightly different nuances.