You can use instance variables and Timer behavior. For example, create "seq" instance variable, pick 5 random circles, assign them seq values from 1 to 5.
Start a Timer for these picked circles for (2.5+Circle.seq*0.5) seconds.
This means that the timer for the first circle will trigger in 3 seconds, the timer for the second circle in 3.5 seconds and so on.
Check out official examples in C3 to learn how to use Timer and other features.