This is because Taiwanese Hokkien doesn't have its own official written form. It has a romanization, but none of the Taiwanese know it, despite most speaking the language fluently.
So when they want to express something in Taiwanese while writing, they would either use ZhuYin 注音, or for the more common words, Chinese characters that sound similar in Mandarin, chosen randomly but turned into a kind of standard slang.
甘蝦, 多蝦 are common (感謝, 多謝), there is even a song by Mayday, "金多蝦" (真多謝).
Other examples include 拍寫/拍謝 (歹勢) "sorry, excuse me".
三小,沙小 is a very rude way of saying "what", a bit like wtf (啥潲).
Some more examples here.
It's just another internet slang phenomenon, not only Taiwanese words get this. 三Q = thank you, common to both Chinese and Japanese, or romanizing random words hen=很 (you'll see things like 我hen開心) or der=得, sometimes derrrr.