People are more likely to ask:
How difficult was that decision?
How difficult was that decision?
without adding to make, although it's possible if slightly unusual, as in:
How difficult was that house to build?
How difficult was that house to build?
It's NOT correct to add to be made. Example 3 is wrong.
Example 2:
How difficult was it to make that decision
How difficult was it to make that decision
is perfectly correct and idiomatic.
