okay, if you look at the guide, while u do see the big symbol with the heartless crest, nobody crest, and spikey cross circle thingy, that each individual enemy in the beastary has the symbol that represents them, and all the non heartless/non nobody enemys have that symbol, because they arent anything but baddies. I think the 3rd symbol is sora's crown because if you hear his strokes (trust me, sounds weird but you can kinda tell) and look at his hand move it looks like he is adding a flat bottom to the curve. Also, if that is the symbol for the new group in the next installment, than its probable they have something to do with the keyblades creation because the kingdom key has that symbol on it. I dont know, however, if sora's crown necklace and chain and mickey's symbol on the kingdom key have a meaning or are artistic licence.