How to run nodes in a cluster on different ports at run time?

i'm running three nodes on same machine and i did not do any settings to ports to each node ,i just clustered using zen protocal?
when am doing load test using JMETER throughput has decreased drastically (around 30/min) ?
i want to know where i am doing wrong?

Don't run 3 nodes on the same machine. It's not realistic.

is it the cause for decrease of throughput?

You are sharing the same physical resources on 3 nodes and add more complexity.
My guess is yes.

If you want to run a performance test, do it in a realistic way otherwise your numbers won't tell you anything.