How to calculate number of nodes of how to monitor when increase the node

Now have 3 node (All is master,data) with 160GB data and data increase
around 600MB every day percentage of job is 50% insert | 50% update. Now my
cluster a bit slow. My question is 3 nodes is enough or how many nodes is
enough. and how to calculate number of nodes

my server spec

CPU Intel(R) Xeon(R) CPU E3-1230 @ 3.20GHz
RAM 32GB
HDD 1.7TB
JVM heap 18GB

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