Did you interrupt it while it was resizing on the first boot? You should be able to create it with fdisk and then format it from the command line. Then you would mount it from the web interface.
I can see two partitions in File Systems: one vfat (62 MiB) and one ext4 (3.52 GB). I thought OMV automatically created a third partition with the remaining free space.
Please provide output from 'cat /proc/partitions' logging in through SSH. OMV's user interface lists only partitions with a filesystem on it on the boot media.
Could this be a bug?
Maybe (from a user perspective) but when preparing the new Raspberry image I chose to not format the 3rd partition with ext4 by intention. We're now running a pretty recent kernel on Raspberries (4.9.x) and since only RPi 2 and 3 are supported any more using btrfs here is an interesting option (especially when combined with transparent file compression).
So all that's missing is a 'mkfs /dev/mmcblk0p3' (me prefering mkfs.btrfs) and then you can add it in OMV UI.
BTW: When thinking about this I also thought about suggesting an UI improvement: Showing available partitions on /dev/mmcblk0 even if there's no filesystem on them. But then forgot about this... 
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