There's 2 things: VM placement during VM start, which is decided by XAPI (the host toolstack). Due to the amount of resources, there's no reason to spread the boot on different hosts. This might change depending on various factors. VM balancing while it's running, that's for XO load balancer plugin. But there's also some rules to have enough resources usages to trigger In other words: everything is normal in your case.

@olivierlambert You are totally right. I 'thought' I had added a firewall rule to allow incoming NTP requests, but I added it again and reloaded and now all three hosts are syncing. Thanks!

For those coming here from Google who are still unsure of some of the details, including what the static lower bound is actually for, here is the corresponding documentation for XenServer: https://docs.xenserver.com/en-us/citrix-hypervisor/vms/vm-memory.html

Thank you all for this wonderful information.