Final answer:
The correct filler words for the sentences are 'are going to spill', 'is falling', and 'a few' based on the context and continuous action at the moment of speaking.
Step-by-step explanation:
The choices for the blanks in the given sentences are:
- Be careful! You are going to spill your coffee.
- Look out! The kid is falling off his bike.
- There were only a few vacant seats on the train.
Understanding the context and the structure of each sentence helps us to choose the correct words. In the first sentence, the construction 'are going to' is used for predictions based on evidence at the moment of speaking. In the second sentence, 'is falling' is the present continuous form used for actions happening at the moment of speaking. Lastly, 'a few' is used to describe a small number of items that is more than one or two, but not many, which seems to fit the context of the third sentence.