Modals for predictions & decisions (will, be going to, be about to)

ModalsWillBe going toBe about to
UsesSpontaneously making a decision at the time of speaking (positive/negative),Making a prediction that is 100% certain (positive/negative)Expressing a decision that is made before speaking (a plan made in advance). 
Making a prediction that is 100% certain (positive/negative),Asking about the future
Expressing an action in the near future.Making a prediction that is 100% certain (positive)Saying something has no chance of happening (negative)
FormalityFormalNeutral / InformalNeutral
Examples
Making a decision at the time of speaking  (positive):
I will go to the bathroom before the next conference. (I am making the decision now) 
Making a decision at the time of speaking  (Negative):
We will not be at the meeting. (I am making the decision now)
We won’t be at this meeting. (I am making the decision now)
Making a prediction (positive):
We will finish the project this week.
Making a prediction (negative):
We will not finish in time.
We won’t finish in time.
Expressing a decision that is made before speaking:
I am going to the concert tonight. (I made the decision in the past, now I have a plan.)
Making a prediction that is 100% certain (positive):
We are going to make more money next year. 
Making a prediction that is 100% certain (negative):
We are not going to make more money next year. 
We aren’t going to make more money next year.
Asking about the future:
Are they going to sign the documents?
Expressing an action in the near future:
I am about to send you an e-mail. 
Making a prediction that is 100% certain (positive):
He is about to lose the sale. 
Saying something has no chance of happening (negative):
I am not about to lose the sale because of him.