You've figured out correctly. The PS3 can only do AC3 in streamed or from USB media. It takes a disc (AVCHD or Blu-ray) to put the PS3 into the other player mode which supports DTS and other lossless codecs.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris