Convoy effect is like you are going through road by car and suddenly you have to stop and wait for few minutes because minister have to pass through that road . This is convoy effect ..where a high burst time cpu bound process is executing and low burst time I/O bound process waiting in ready queue ..
all these given scheduling algo - FCFS, Priority and SJF are Highest priority First Scheduling algorithm means high priority process execute first then low priority process can execute.. so low priority process is starving .
FCFS is non preemptive, FCFS assigns priorities according to their order of arrival or arrival time. Non preemptive Shortest-job-first (SJF) assigns priorities according to the burst time. In preemptive shortest-job-first the scheduler is always running the job with the highest priority. hence in all 3 cases convoy effect possible.