I am a native English speaker who is learning Mandarin Chinese for the AP Chinese test next year. I stumbled on this "anomaly", and I would like your help. Thank you.
That 三明治 came from transcribing the English word Sandwich into Chinese. That is to say, it is meant to approximate the pronunciation of the English word. You aren't meant to interpret the individual characters literally.
As Stan points out, it is sometimes transcribed as 三文治 as well. Historically it has also been written as 三味治, but that's pretty much extinct.
It's just a transliteration from "sandwich", does not have real meaning.
BTW: It's usually called "三文治" in Hong Kong and Macau.
I couldnt comment because I dont have 50 reputations but I have something i must say so I'll just post an answer.
Stumpy Joe Pete:
Since you are talking about in general, there are times education level and the result of borrowing word is relevant. I know linguists tend to consider more linguistic aspects of things, but this is just life and things aren't perfect. someone made a bad or arbitrary decision and people starting to use it, then it became a standard, such things happen. (hint: what are you typing with? Dvorak?)
Of course, this is not the case. Alex's comment might be a bit offensive, but you should try to respond professionally.
Edit: In no way I am saying one is more linguistic than other, that is for linguistic people to debate. I am a native speaker of Chinese, and I am simply commenting which practice I would prefer and why. Borrowing words by sound give new meaning to old characters, this becomes burden of memory and communication when it reaches a certain level. Sounds does not make sense on its own, unless you already know the roots in other language, but a translation by meaning would, to some point.
I do understand there are many today's common expressions are or partially are translated by sound (such as 苹果 for apple), it is not the case I'm against any of those or more of those. Some stuff are just hard to translate I agree. It is simply the case after arguing with people I still have to use this language every day, and I want it to be something I understand more easily if possible.
English is common for today's Japanese people. Have you heard them pronouncing it? I think its because the difference in sound is large and whether allowing closed syllables. The fact Chinese is strictly one syllable per character also played some role.
for the question:
Since nobody mentioned it. One possible reason this question is asked is: in mainland China, we strive to use less pronunciation based translations. Examples:
the left ones are what thought to be more "official" "written" "professional" in mainland china.
Chinese characters are meant to be the meaning of something mostly. We strive to reduce words like 三明治 is because having more translations that are mere symbol of sounds breaks the system.
Adding my 2 cents...
sand-wit-chi will be used to remember how sandwiches is prounced, and "sand" sounded similar to "三" and so on. These terms are then eventually became the item's name.