How does a 3-speed automatic transmission work?

There is a website about how a 3-speed automatic transmission work at http://leakylugnut.com/driveline/automatic-transmissions/ but I'm confused about how first gear work. Is the rear carrier splined to the output shift or is there an one-way clutch between the rear carrier and the output shaft?

And what transmission are you asking about?

Turbo-Hydramatic 125 (FWD) up to TH425. I have a shop manual for 1982 FWD X-Body cars. I need clear diagrams of the parts inside the transmission.

Go to Sonnax.com for good stuff.