3-speed
to say all three speeds are weak is quite an inexperienced generalization.
there are a few different three speed models that were made.
The a-745 is a weak transmission because it was intended for very light duty and was operated as a column shift unit only. It is easily recognized as a top loader.
The a-230 is a much better unit and it was made by new process which is same manufacturer of our beloved 4 speed. It came in fully synchronized heavy duty versions and non- synch versions for slant sixes and/or light duty. By 1972 you are going to have a fully synchronized a-230 that is going to be more than capable of holding up in the application.
The a-230 is recognizable because its a side loader and basically looks like a smaller version of the 833. It also has provisions on the tail shaft for a floor mounted shifter.
Granted a 4 speed has its advantages in performance with closer gear ratios, but the three speed is also a good unit and will serve its purpose.
Three speeds helped with insurance rates as mentioned above. There are even Mr. Norms cars in existence with the a-230 three speed in place.