The answer to this is Option 3.  The only one of these expressions that Kenny uses at all in the book is “Ready, Aim, Fire.”  Kenny uses this expression five times in the book (according to a search of Amazon’s searchable copy of the book).  He does not use the other expressions at all.

To be more precise, Kenny always uses this expression as part of a longer simile.  He always thinks to himself that someone “might as well have” tied someone else to a tree or pole and “said, ‘Ready, aim, fire.”  This is a simile that Kenny uses when he thinks that someone is being put in a bad situation.  For example, we first see this expression in the first pages of the book.  The family is in Michigan, listening to the weather report.  It is terribly cold.  Momma does not like living in Michigan because she says it is “a giant icebox.”  She is from the South and prefers the climate of that region.  In other words, there is clearly some tension within the marriage because Dad has brought her to live in Michigan.

The weather reporter talks about how cold it is going to get in Michigan.  It might get to be negative 20 degrees.  Then he says that the temperature in Atlanta will reach the 70s.  This is when Kenny thinks that

The guy might as well have tied Dad to a tree and said “Ready, aim, fire.”

In other words, the forecaster has gotten Dad in trouble, in this case with Momma.

Kenny’s use of this simile is one way that we can see that he is a creative and intelligent boy.