Let me quote https://docs.docker.com/engine/swarm/admin_guide/ 
### Maintain the quorum of managers
If the swarm loses the quorum of managers, the swarm cannot perform management tasks. If your swarm has multiple managers, always have more than two. To maintain quorum, a majority of managers must be available. An odd number of managers is recommended, because the next even number does not make the quorum easier to keep. For instance, whether you have 3 or 4 managers, you can still only lose 1 manager and maintain the quorum. If you have 5 or 6 managers, you can still only lose two.
Even if a swarm loses the quorum of managers, swarm tasks on existing worker nodes continue to run. However, swarm nodes cannot be added, updated, or removed, and new or existing tasks cannot be started, stopped, moved, or updated.
Quorum (=majority of the node members) requires at least (n+1)/2 active manager nodes.
For such a small cluster, I would use 3 manager nodes.