this is possible, but one has to ask what are you trying to achieve? If you only need 30 Cores (8 + 8 + 14), then a standard 1/8 rack is fully capable of delivering that, without having to add additional DB servers.
One thing to consider in your 8 + 8 + 14 config: If you are creating a 3-node RAC, then you will lose almost 50% of your compute resources should the 3rd node fail.
That is very much possible and a common scenario when people add newer generation DB nodes to an existing Exadata rack. With each new generation hardware, number of cores keeps going up. So if the new nodes are getting added to the same RAC cluster, nodes are going to have different number of cores.
In a 3 node scenario, you need to think about the cases that Andris mentioned above.