Questions regarding CPU scheduling algorithms ( Round Robin and Priority scheduling )
You had to make assumptions. Your problem is that “priority scheduling with round robin for equal priority” is not enough to specify exactly what happens. 

Your scheduler will consider a change in the schedule when the running process finishes, when a new process is started with priority same or higher than the currently running process, or if the quantum is elapsed - but it’s not obvious if this is every three seconds, or three seconds after the last change in schedule. And exactly what “round robin” would pick isn’t clear. 

You’d need the exact details of the scheduling algorithm.