Use these tables as a guide.
D200 - Camera Raw 3.3 - compatible with CS3+
D300 - Camera Raw 4.3 - compatible with CS3+
D300s - Camera Raw 5.5 - compatible with CS4+
So you cannot open Raw files from the D300s in CS3.
Your choices are either to upgrade or download the free Adobe DNG converter, convert the Raw files to DNGs then open the DNGs in CS3.