Can't get P3 spot instance on AWS
Spot Fleet aims to meet your capacity requirements across a range of instances. This reduces the risk of any unavailability of a specific instance type or family affecting your workload.

As you are interested in only one specific machine type, you will need to remove the other types from your spot fleet request; or just create a separate spot instance request for the p3.2xlarge (a plain spot instance request, not a spot fleet request)