I do not know much about automatic transmissions, but I'll try to give you some ideas.

For your speedometer dying on you, it's possible that the speedo cable has worn away and isn't getting a signal. Or that one of your wires came undone. What year is your car? If your car stopped receiving a speed signal then this could have caused your ECU to become confused, causing the blinking light in S3.

This same problem, not receiving a speed signal, could have also caused your CEL to come on. Get the blink code and see where that leads you.

Does the speedo jump? Or not move at all?