You sure that the TV has not got an optical output? The next size up does, but i cannot find anything about yours having an optical output (which is odd for a fairly recent model!) If you are 100% sure, then the only way from what you have to hand, is using the analogue audio output (small 3.5mm jack on rear) If it has one. I have found some specs that state that it has. You could use a 3.5mm to 3.5mm jack lead form the TV to the amplifier on your sony HT-RT3.
Hope this helps.
K.