I'm setting up my first Eldritch Horror solo game (base set only) and just noticed something I hadn't either seen or taken into account before. What does the number (1-3) on the bottom right corner of the Expedition Encounter cards indicate? I see there are 3 for each Expedition location, so perhaps not a difficulty progression, but just an enumeration for clerical purposes?

To clarify: would we shuffle the 3's, then the 2's and stack those on the 3's, then the shuffled 1's on top?

All the encounter cards in Eldritch Horror are numbered, I don't believe this serves any purpose other than uniquely identifying the cards since many times they will have the same artwork and title but different text.