Note that IS clustering does not provide load balancing. So you’d need to direct your traffic amongst the 2 or 3 nodes in some way.
The performance increase of additional nodes depends on a number of factors. How all the components in the solution are configured. What the integrations are doing. There isn’t really a rule of thumb that indicates how much more work can be done by adding another IS instance.
I’d advise that you study the IS cluster documentation and get a good understanding of what IS clustering does and does not do. It is common for people to assume that there is more capability built-in than is really there.
A year or so back, there was a thread on “IS clustering to be avoided”. Many of the items there still apply.
A load balancing cluster is probably sufficient for most needs, and that doesn’t require IS clustering though you may want it for some functions (scheduled tasks, for example). A Broker client cluster can also be useful, where multiple IS instances connect to the same Broker and a queues (IS clustering not required for this either).