it took me a while to realize that Regice, Regirock and Registeel's 7 eyes were representing their trio number.
Articuno, Zapdos & Moltres are the first three and Suicune, Raikou and Entei are the other 3 which makes them 6 in total, So the golems are the 7ths.
Also I didnt know that the order the Legendary Beast in the Pokedex comes in the same order the Burned tower happened. Lightning stroke it(Raikou), it got set on fire because of the fire(Entei) and then rain stopped further fire(Suicune).