Does any1 know about a 3speed trans. w/ overdrive?

i have a 1968 chevrolet c10 pickup with a 3 speed on the column, overdrive on the dash factory. i go down the road, im in 3rd gear try to push clutch in and push in overdrive which is on a cable. it wont work. ive got power to everything. just wondering if anyone knows anything bout this transmission. thanks!!!!