If I understand correctly, the high memory usage alerts is coming from the Windows 2012/08/03 VMs and not from the VMware ESXi hypervisor itself.
There are a couple of ways to handle this:
(a) increase the memory (i.e. virtual RAM) allocation to the VMs, for example, if current allocation is 8GB and you can increase to 12GB (if you ESXi hosts has enough free)
(b) increase the pagefile.sys sizes of the Windows server VMs
(a) might be harder to do especially if the VMs RAM allocation are already very high and hitting close to the ESXi host total RAM.
For (b), you can set it manually and set it up to 3x (depends on OS version) of the RAM allocation. The cost to this is disk space. If increasing the pagefile.sys size causes the free disk space threshold of the VM to be breached, you obviously have to increase the disk size of the virtual disks of the respective VM(s).
Have a look at this
https://support.microsoft.com/en-us/help/2860880/how-to-determine-the-appropriate-page-file-size-for...
Obviously you could do both (a) and (b). And if you do (b) first and (a) later on a VM, you might have to revisit (b) as the maximum size allowed would be different. You could also selectively do (a) on certain VMs or (b) on certain VMs, and both (a) and (b) on certain VMs.
Hope that helps.