Taking cues from the sparse information provided, a terse reply:
1965 and later Saginaw 3-speed transmission had synchronized first gear.
It and the correct bell housing will fit the 235 but the transmission output is open drive (not torque tube).
There are 1st gear ratio options but final gear is 1:1. This you need to know to work with your current rear axle ratio.
To avoid calamity of a modified 3-on-the-tree setup, you would want a Hurst truck floor shifter, modified bracket and linkage. You have to make a new hole in floorboard.
Others will come along and give recommendations on the T5 5-speed transmission from an S10 but you specifically stated a 3 speed transmission of unknown year or make.
