If the machine is an i7 it likely only has 4 cores/threads total. 3 VM x 2 cores = 6 cores required. Also, instead of having 3 VMs with at least one IO controller and a NIC each, you have 3 VMs sharing 1 IO controller and 1 NIC. Not as fun as it sounds.
My lab machine is a HP z820 with 32 cores and 128GB of RAM. When I spin up a SmartStart style 3x3x3 cluster + 1 bastion host + 1 NFS host (3 masters, 3 infra, 3 app nodes + 2 support hosts), my IO controller can't keep up. I might as well be running on some 4 year old cellphone. Which reminds me, I need to move those VMs over to the storage attached to my RAID controller.