Most likely, you are experiencing a firmware issue. Samsung
has had repeated issues with IR codes sets varying from
one version of firmware to the next. Most likely, the
newest HLTs support the HDMI3 code, while earlier versions
do not. Same issue with the first runs of DLPs(HLM) supporting
discrete power, but not input, then from version 314
and up supporting power/input, then later still supporting
discrete color and aspect.
Even if your purchase was recent, it doesn't mean that
you have the latest version of firmware. The set could
have been stored for quite some time and need to be updated
by Samsung to the latest firmware to support the HDMI3
code.

I don't have a HLT in front of me to test, so this is
purely speculation based on experience.

Jason