Each empty slot of the Keystone Tablet can be placed in 4 letter slots. According to BIP39, the first 4 letters of each word in the mnemonic list are different. That is, four letters can be used to represent a unique mnemonic. If you encounter a 3-letter mnemonic, you can fill the empty space with the blank side (reverse) of the letter tile. You can view the 2048 mnemonic list here.