then rear facing buddy seats only have lap belt .quite usless.
A rear facing seat is much much safer than a forward facing seat in a crash. The 3 point belt was implemented to stop the top half of the body whipping forward from the point where the lap belt would be in a crash.
With a rear facing seat, you are generally sitting up in a crash, and your body is forces into the seat. Crash passengers in rear facing seats receive almost no whiplash injuries (and the survival rate in a plane crash greatly increases for people in rear facers, e.g. the hosties get off alive). Putting a 3 point bely on a rear facing seat would be a bit redundant, as you are unlikely to have a high speed crash while reversing.
The seat looks like this, and they are regularly for sale on german ebay