Welcome to the community.
First of all the term you are using as 'mini cloud' is actually a cluster. You can add all the three servers to single group referred as 'Cluster' (Please don't confuse with Microsoft cluster technology MSCS). This cluster will act like what you said a 'single big machine' (incorrect word to use though) which will hold the total resources of all three hosts. Whenever you create a new Virtual Machine in this cluster the VM can goto any host in the cluster whichever is having enough resources (RAM/CPU etc...) available.
At this point I would recommend you to test this using the vSphere trial version. Once you are happy and confident, you can go ahead and purchase the license accordingly. Trial version will have all the features available for 60 days. Below is the link to the vSphere trial version,
Please let us know if you have more questions.
Can you offer any links to a "How To" for clustering the ESXi servers ? Or is it quite easy and simple to do ?