Bernard

Technically it is possible to have different hardware configuration in an Elastic cluster, as 3-node requirement to avoid split-brain scenario and making sure that there is always a master node.

However there will be impact on the performance of the elastic cluster because less resources in a server. There is no easy answer to this, and benchmarking is the only to measure how this cluster performs.

Please see couple of links that you may find useful on this topic.

How many nodes should an Elasticsearch cluster have? (cpratt.co)

Benchmarking and sizing your Elasticsearch cluster for logs and metrics | Elastic Blog

Hope it helps.

Thanks.