I'm sure this has been answered at some point years ago, but... what happens here?
Buy Sewers. Use Chapel to simultaneously trash a Catacombs and an Estate. You now have 3 pending triggered effects: Catacombs, Sewers, Sewers. Decide to order a Sewers effect first. Use it to trash a Rats. This produces another pending triggered effect.
Which is true at this point?
A) You may order the Rats, Catacombs, and Sewers effects any way you like.
B) You must resolve Rats' effect immediately after the instruction that triggered it, so you cannot resolve the Catacombs or Sewers effects until after that.
C) You must resolve the first set of effects (Catacombs and Sewers) before you can resolve the new one from Rats. (In which case there would have to be some follow up questions to about what that actually means.)
D) Other.
In other words, and more generally, is there only one set of pending triggered effects at any given time, or can there be multiple sets, which have to be handled in a certain order themselves?