1. The Mandarin word for the number three is pronounced "San".
2. In Taiwan, there is an adjective used to describe a food that is pleasantly chewy (i.e. a gummy candy). This word is pronounced and written as "Q".
3. Many Mandarin speakers, particularly in Taiwan, have trouble pronouncing the "TH" sound. Instead, it usually comes out as "S".
Put it all together, and "3Q" magically becomes "Thank You". Given the relative ease in typing these two characters vs. the complete words in either language and you have the shorthand that nearly all Taiwanese use when sending instant/short messages to each other.
Here, you can see our lovely model is holding a flattened dried squid. This is in our local grocery store - similar to Albertson's in some ways and VERY different in others...