Changeing a Transform var

First, this would have been easier to read if you had selected all of the code and then hit the "zeroes and ones" (i.e. code) button.

Secondly, try explicitly typing your object, like:

var instantiateObject: GameObject;

The other issue might be that you try to instantiate your object before you've selected which object it will be (you hit the Instantiate "if" before you hit the Select "if"s). If your code above is in that order in your program, you might want to reverse it. Personally, I don't know why you wouldn't just select and Instantiate at the same time:

if (fireRequirements){
    var clonedObject: GameObject = objectDefault; // set which object you want if none are selected.. i.e. the default
    if (spawnObjectA){
       clonedObject = Instantiate(objectA, transform.position, Quaternion.identity)
    }
    else{
        if (spawnObjectB){
           clonedObject = Instantiate(objectB, transform.position, Quaternion.identity)
        }
        else{
           if (spawnObjectC){
              clonedObject = Instantiate(objectC, transform.position, Quaternion.identity)
           }
        }
    }
}      

Another hint: variable names normally begin with a lower case letter and functions with an upper case letter (see my code). This isn't required, but it will help you at a glance (if you see "IsSpawned" you will know you forgot the "()" for the function, instead of not being sure you didn't mean the variable "isSpawned").