Do we need a multi-Visa or single Visa for the following 3 week trip?

You need a multiple entry tourist visa as you will enter China 3 times, one entry for Beijing, one for Sanya, and one more for flight to Beijing from Thailand. But please note that here for the last entry, if you have flight ticket from Thailand to Canada, which makes Beijng as a transit, then that entry is not necessary, you can use free transit policy. A double entry is enough.