Elemental Chest, it is possible to wait and have 2 or 3 elemental chests active at the same time
Someone who knows more than I will have to answer regarding the waiting and 3-at-a-time, but I once had two show up simultaneously. I had one on my Hero chest slot, and filled my titan chest, which turned into an elemental chest as well. Maybe it is a random frequency for each chest, not overall.
In a year of playing, this has only happened once.
I believe it is random frequency for each chest, with Raid chests having a slightly higher probability than monster chests (not sure of the odds of Titan chest relative to the other two).
That being said, I never wait to start filling elemental chests as there is lost opportunity costs in doing so.
Gotta be a contender for the longest odds in the game