Yup, true. Ho-Oh did revive three Pokemon at the Bell Tower when it burned, not the Brass Tower. Eh, close enough.
Celebi and Dialga both time travel, so they couldn't create each other. They both exist for infinity, therefore if one created the other, it would elevate them to the status of Mewtwo/Arceus. But, since Dialga is the titan of time, I see why you would think that.
According to official lore, this is true. But, upon closer inspection, the tower burned to the ground, right? Anything caught inside that building would be dead, and the body would be entirely incinerated. Ho-Oh wouldn't know where to look, unless the pokemon caught inside were fire-types who were immune to fire. So, my theory is that all three were sturdy fire-types who could also take a collapsing building on top of them. This brings me to the theory that all three were originally Heatran, which is the only viable option besides Macargo. Heatran is sturdy enough to fit the description, and very fiery. Heck, it melts its own skin off. So, our most viable option for the trapped pokemon in the tower is Heatran. Macgargo does make a close second, but only because Macgargo is a snail. Snails can retract their bodies into their shell, and in Macgargo's case, a stone shell. That would allow Macgargo to survive a building falling on it, and since it is part Fire, it won't get burned. So, Macgargo would have lived, but Heatran would have died. This leads us to the conclusion that Heatran was the only Pokemon that fits this description. Thank you for mentioning that.
