Vyxal, 84 bitsv2, 10.5 bytes
3ḋ'1⋎⁰cnÞ⇧*;LPorts user1609012's python answer, so make sure you go upvote that. It's a much simpler answer than what I would have come up with.
Explained
3ḋ'1⋎⁰cnÞ⇧*;L3ḋ # Get all combinations without replacement of length 3 from the landscape' ; # Keep only combinations n where: 1⋎ # The combination with the middle letter removed⁰c # Is in the list of landmine flags * # And nÞ⇧ # n is sorted in strict ascending order L # Get the length of the valid combinations💎Created with the help of Luminespire.