Ive had 4 or 5 of 10 buildings ask for the same thing before. As you get more and more finished on your way to 160 for the gold, they try to make it harder on you. When I used to save up tons of rare items for the epic challenges, that's usually when it would ask for 5 corn in 3 different buildings. It was very frustrating.
There are great tutorials here and on the Reeddit page about epic tasks. My advice is wait until you have at least 6 buildings with 3 epic points each (preferably more). It's a lot of gold keys, but it gets a lot easier when you have more options. Also store any 1 or 2 point buildings before starting an epic task. I ended up only collecting the 3 point ones, and focusing on one type of epic for a few weeks or months until I had enough landmark or mountain 3 point buildings to start.