Maximum retaining wall height
This is about load bearing and claims. The pressure associated with 2 ft of dirt is probably well within the load bearing specification of the blocks in question. 3 feet may not cause any issue, but you cannot hold the manufacturer liable if it collapses and/or if someone gets hurt.

If you exceed the manufacturers specifications an inspector might have a problem with it too. I would recommend adding weep holes, french drains, and possibly a deadman. And since much of the weight comes from moisture, the backfill could be gravel, to improve drainage. In fact there are lots of ways to shore-up a retaining wall, but the onus would be on the "engineer" if something were to fail.

Here's a good picture of some retaining wall features. Source: http://www.fairfaxcounty.gov/nvswcd/youyourland/soil.htm

  

Edit- A tie back is used on block walls. Screw types and anchor plates are common, but there are probably many ways to accomplish the act of tying or holding the wall in a similar fashion.