Adding some more detail to @PlantedRich
's info on what the actual source of sodium in softened water is, below is a way to calculate the burden and potential effect. TDS meters can help here, as well. However, if your pre-softened water is truly 3 dGH, then you may not need a water softener, as that is quite soft already. Your note that the softener uses very little salt also indicates that you may not need a softener. Further, if you have switched to potassium chloride (which is expensive as compared to sodium chloride), there would be none of the possibly harmful sodium involved anyway. If you ever decide to go back to sodium chloride, here is how to estimate sodium pass-through from the softening process:
Sodium added by a water softener is a function of total grains per gallon of general hardness.
Water softening references are usually GPG (grains per gallon) of GH, which is close enough to dGH to be equivalent. When softened, the GPG of GH is converted to the sodium component with this formula: GPG of GH x .46 = sodium GPG. to convert the sodium GPG to ppm, use this formula: sodium GPG x 17.1 = sodium ppm.
So, your dGH is 3, which is 3 x .46 x 17.1 = ~24ppm sodium. If you do a 50% w/c weekly, that 24ppm will stabilize at ~47ppm in a few weeks. In hydroponics, sodium levels of 30-100ppm are considered toxic, depending upon the plant.
As you can see, it is better to not use softened water. It may be better to have the 3 dGH (54ppm) in Ca and Mg than it is to have the 47ppm of sodium. I don't know if the potassium contribution from KCl softened water would contribute the same ppm of K as is the case with NaCl. There is a good freshwater potassium test kit available from Salifert. You may want to try it to see how K you are adding weekly.
Incidentally, adding Seachem acid base is a very short term solution to the alkalinity issue and is somewhat unstable. The best way to address all issues is with an RO unit, although some members have been playing around with more powerful acids.