You're caring for three children in your home: a three-year-old, a four-year-old, and a two-month-old infant. the infant often become hungry and wants to be fed just when you're involved something with the two older children. what should you do with the infant

In this situation, you should  Hold and feed the infant while you supervise the activity of the older children. 
Three-year-old and four-year-old children have already entered the stage where they could explore the environment around them on their own without direct help. So, you could step back and feed the youngest infant while watching the other from afar (only step in if they're about to do something dangerous, such as swallowing little toys)