If you can pick up the SA-3 for a good price I would jump on it.
Both the SA-3 + 400a are class A amps. The Forte 3 being class A/B.
The newer version of the model 3 was the Model 6 and the Model #6 can be Modded to run in pure Class A, as the Model 4 pure Class A and #6 are pretty much the same amp. So converting it is not to much work at all.
I am not sure if your Model 3 can be converted to Class A.
But you can ask Jon Soderberg at http://www.vintageamprepair.net/
if it can be done.
Jon worked at Threshold with Nelson Pass and does upgrades, rebuilds and Modds to Threshold + Forte amps.
A year os so ago the convertion from the Model#6 to Model#4 was about $150. US plus shipping and if you have some skills, Jon will even tell you how to do it. This is the way I went and so saved on shipping costs, as there wasn't much to the Mod.
The Class A version of "my amp" is far, far better the the class A/B version, easy. even though the A/B version had 4x as much watts.
Many think the SA series are the best of Nelson Pass, even better then the newer Pass Labs, some will say.
I've used Threshold and Fotre amps with a few different Planner speakers.
Maggies and Eminent Technology LFT 8A's with great results.
You can't go wrong with Threshold of Forte. Great Amps!!!