TL;DR;
You can even start with a single node, but you loose the highly available factor of c*.
Cassandra is built for systems that handle huge volumes of data, terabytes and in some cases petabytes. Many users typically switch from MySQL (and lots of other RDBMS) to Cassandra once they find that their current DB system can’t handle the data load efficiently (querying gets slow, managing storage becomes challenging etc.)
Why 4-8GB gb of ram?
The 4-8 GB of ram is to do with the JVM and the size of ram on efficient garbage collection. The advice is stating not that you should start on 8 GB, but hat you shouldn’t have more than 8GB
This doesn’t mean to say that you cant use Cassandra to start up a single node on a very basic machine (some people actually have cassandra running on a raspberry pi).
Why do people recommend 3 nodes?
Availability is one of cassandra’s main selling points. If you have 2 nodes with RF=2 then you cant perform writes if a single node goes down. If you have 3 nodes you can still perform both reads and writes.