SQL Server 2014, Volumes in Resource Governor
It seems reasonable to assume, per the documentation and indeed the names of the parameters themselves, that MIN_IOPS_PER_VOLUME and MAX_IOPS_PER_VOLUME control the maximum quantity of I/Os per second per volume.
i.e. as in your question, if you have three volumes in the resource pool, and set MAX_IOPS_PER_VOLUME to 300, then you'd have a maximum total IOPS of 900.
Without source-code-level access to SQL Server, it's impossible to say for certain if that's how it actually works, however the documentation says (emphasis mine):

MINIMUM_IOPS_PER_SECOND:
Specifies the minimum I/O operations per second (IOPS) per disk volume to allow for the resource pool.
MAXIMUM_IOPS_PER_SECOND:
Specifies the maximum I/O operations per second (IOPS) per disk volume to allow for the resource pool.