Is it a PS3 Slim?

If so, you need to set the HDMI output format from Linear PCM to Bitstream. Also ensure you run the HDMI auto-detection routine so that your receiver can tell it what audio formats it accepts.

If it's not a PS3 Slim, then you can't output Dolby True HD or DTS-HD Master Audio (due to an HDMI chipset limitation). However that's not actually a problem, as the PS3 will internally decode these formats and output them as 5.1 or 7.1 linear PCM over HDMI to your receiver. Whether your receiver decodes the original HD audio or whether the PS3 does and passes it as L-PCM, the end result is absolutely identical: perfect, lossless audio reproduction.

In fact my preference is to let the PS3 do the decoding, since it's one of the only players that can properly mix BD lossless audio with supplementary audio for commentaries, menu sounds and so forth.