We present a simulation-based study of the impacts of different types of attacks in mobile ad hoc networks (MANETs). We consider three common types of attacks: blackhole attack, jellyfish attack and neighbor attack. Specifically, we study how the number of attackers and their positions affect the performance metrics of a connection such as packet delivery ratio, throughput, end-to-end delay, and delay jitter. The results enable us to suggest measures to minimize the impacts of the above types of attacks in MANETs.