To be entirely accurate the B3 chipset revision was for both H67 and P67 platforms. When the Z68 was released to market the B3 revisions were incorporated into alll of the later released motherboards.
+1 Chrsteracorgi is right.
When the H67 and P67 chipsets were release they had a B2 stepping for the Sata II controller. This SATA II controller over time would degraded and lead to data corruption that lead to us bring them all back in. When they were re-released most of the motherboard manufacturers would tag all of these boards with a B3 to show that they had the updated chip. So by the end up March all the old boards were off the market and the new replacement boards were out on the market. When the Z68 chipset was release in June there never was a question of it having a B2 stepping chip because all of these chipset were after the B3 release.