Which topology are you deploying? For a standard 3-node cluster the minimum instance should be at least a c3-standard-4. For advanced topologies you shoud consider at least a c3-standard-8 (or the equivalent depending on the availability within the region your are deploying). Ideally, you should also consider ssd for the disk. The larger your anticipated workload, then the larger the instance should be, and should primarily be determined by your MySQL Database needs.

How large will your database be? By default the instance is launched with a 40Gb partition which should be suitable for most small to medium workloads. You should consider increasing this as required, based on your MySQL Database needs, taking into consideration that binary logging must be enabled and there will be an almost 1:1 relationship between binary logs and THL Logs.

Standard Clusters : You will need a minimum of 3-nodes, for any greater, there must be an odd number to maintain quorum, for example 3,5,7,9 etc. All nodes must be in the same region however they can, and probably should, be in different zones.

Composite Active/Passive & Composite Active/Active Clusters : These topologises consist of multiple clusters connected together, each cluster must conform to the same node count and rules for a standard cluster, and there must be a minimum of 2 clusters. Therefore for these topologies you will need a minimum of 6 nodes (3 per cluster). Each cluster may reside in a different region, allowing you to configure full geo-distribution.

For more details and to understand the different topologies, see Deployment: MySQL Topologies. Whilst the launch wizard on the launched VM will assist you to pre-configure either a Standard cluster, a Composite Active/Passive cluster or a Composite Active/Active cluster, users with advanced knowledge can adjust and configure the nodes to suit any required clustering topology available.