I may be misunderstanding your question, but when you request a catalog item, you select the number of machines you want to build. Can you not just set this to 2 or 3? Of course the limit of the number of machines may be limited in the blueprint using the "Maximum per user" setting
I know that it is not possible to request more than one VM in case you have custom property Hostname set as required. As you can insert only one hostname, the option to request more machines is hidden automatically.
Is it possible that this is your case?
depending on how custom your deployments are you can either populate the number of machines in the IaaS blueprint. If your deployments are customized, what i do is use a CSV to spin up VMs through the vRA REST API.