make sure all three are on the same level
make sure they are all within range of each other (you can do this by applying a redstone signal to the "middle" landmark)
make sure when you want to connect the land marks that you right-click the "middle" landmark
note: the "middle" land mark is the one that can see both others in straight lines.
The first 1 made perfect sense but the last 2 got me confused
No worries, hopefully this helps for 2 and 3.
2. make sure that the two landmarks aren't too far away from each other, that is, further than 64 blocks away. try moving them closer and see what happens
3. make sure you are right clicking the correct land mark
You should have three landmarks, marking three points of a triangle
1
x
x
x
x
2 x x x x x x x x x 3
in this very poorly drawn example (drawn?)
you would need to right click on the landmark at point 2 because it can see both landmarks 1 and 3 in straight lines
this would then mark a square (or rectangle) with the distances between those landmarks.
I hope that is a little more clear and that I didn't explain what you already knew.