Wouldn't any number of XEM suffice then instead of the 3 million? The work alone discourages the setup of multiple nodes. Only wealthy people can join the node reward system with this setup.
In my eyes even people with no money should be able to setup a node.
And I think the aging and the testing of the nodes also protects the creation of multiple nodes from one person.
Someone could setup 100 of nodes and cash out pretty well. You need a minimum. I agree that 3 million is too much though.