A friend owned a Chevy dealership back then; I have copies of the '67-'69 Camaro paperwork.
3 of the '69 Camaro SS they stocked were 3 speeds. One of them also had a ducted hood and wheel covers, a really odd combination of options.
If the VIN of your car ends in 577, 347 or 970 I could have good news.
A truly rare 3-speed known to the CRG is a documented 325/396 69 Camaro SS, unfortunately no longer in that configuration.