No, FlexRAID is nothing like RAID 3. RAID 3 stripes data across all the drives per byte, with the parity byte on one single disk. RAID 4 stripes data across all the drives on a block level using a single disk to store the parity block.
FlexRAID does block-level parity like in RAID 4, not byte-level parity in RAID 3... So it's closer to RAID 4 than anything else.
I've never seen RAID 3 in use anywhere, as it's pretty inefficient. Having to read a byte at a time sequentially from each disk is very inefficient compared to RAID 4 and 5 where they read whole blocks of data in parallel from all the disks at the same time. I worked at a company that built Video-on-Demand servers back in the mid-90's (way ahead of their time), and they preferred RAID 4 over 5 (writes are very slow, but read speed is excellent on RAID 4, mainly because you don't have to skip the parity block [re-seek the head] on reads like you would in a sequential RAID 5 read.) For serving up multiple large sequential write-once-read-many files like video files, RAID 4 is tops.