No problem at all, as long as you avoid the IP addresses already used obviously.
If I understand correctly, the new Vcenter will only handle the 3 new ESXi hosts and will not interact with the production. If so, there are no problems.
I advise you only to reserve a range of IP addresses to be used for the test environment, in order not to create IP conflicts with the production.
Attend to following checklist for your demo datacenter:
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- Isolate Networking of your new vCenter and hosts that you want to use in this new virtual infra (demo environment) include (IP addresses/Subnets and VLAN IDs if you have) to avoid collision/duplication/conflict.
- Separate Storage infrastructure if you need to provide shared datatstore on your demo cluster or use some virtual storage provider (Like iSCSI target or Open Filer). Don't use existing storage systems because you will impose more IO to them.
- If you need Backup, Monitoring or any other related system that you want to use for this new virtualization, install temp and fresh servers is recommended and don't use existing provided solutions.