Why does rsync spawn multiple processes for me?

http://rsync.samba.org/how-rsync-works.html

Rsync is heavily pipelined. This means that it is a set of processes that communicate in a (largely) unidirectional way. Once the file list has been shared the pipeline behaves like this:
generator → sender → receiver

The output of the generator is input for the sender and the output of the sender is input for the receiver. Each process runs independently and is delayed only when the pipelines stall or when waiting for disk I/O or CPU resources.

You are running a local rsync (source and destination are local filesystem) so all three processes will run there.
There is nothing you can do, this is by design.