Ok, I don't know which model of PS3 you have, but 3D mode on those units have always required a software download to turn that on.
The last generation of PS3...I honestly haven't seen those yet, so I'd have to guess if it needed the software or not.
But let's cover the basics.
Let's assume the PS3 is a couple years old and needs the software. It's not an automatic update. You do have to go to Sony's website and download it.
Next, you cannot use any HDMI cable. It must be an HDMI 1.4 cable to transfer the 3D sync signal from the PS3 to the TV set.
You do have to have your PS3 set to 1080P or 720P. You need the double-framing to get the left and right eye pictures transferred, and only the P(rogressive) frame rates do that.
I have a couple of pictures that shows you the different cables and the different TYPES of 3D. A key point....when 3D was first created, you had to MANUALLY turn it on, and on the older DVDs, that was needed. Newer DVDs are automatic turn on.
Avatar is a newer DVD, and you'll notice that in these pictures I post.
If you have the basics....then it's easier to Master 3D.
Believe me, I had my fun with it when it first came out. One thing I learned, If it doesn't work after making the needed changes....turn the units off, restart it, let it power up with the new settings. Mitsubishi TVs require that, but I don't think Sansui uses the Mitsubishi chassis.
Ok, here is the table for the HDMI cables --
And here is the various types of 3D and the required setups/signal sources --