Equilibrium I have used myself and it seems to work with no other issues, as far as I can find out. However, it does get expensive since obviously it has to be used with every water change [I no longer use it]. And in your situation, it actually is at cross purposes to what you want.
If the hardness test is accurate, a KH of 3 is soft, and this means the pH of the aquarium may slowly drop. This occurs in all tanks, where there is nothing to prevent the acidifying of the water which is due to the natural biological processes ongoing in the aquarium. My tap water has a pH of 7.0-7.2 but < 1 dKH and GH, and in my 70g and 90g the pH went down to 5 [I let it, it was not accidental]. I keep the pH in the 115g at 6.2 by using dolomite to buffer the water.
The pH of 7.6 is fine, ideal in fact for all livebearers. But it will probably lower gradually if you do nothing (assuming again the hardness test is accurate). But a change from 7.6 to 7.2 is minimal in terms of its effect on the fish, and in my view the guppies and molly will be fine at 7.6. Raising the hardness of the water will also likely increase the pH, the two are related, which is the opposite of your intention.
I would leave it as is, but monitor it regularly, at least once a week before the partial water change, and if any change in pH is evident, monitor it daily. Always test pH at the same time each day to obtain a more accurate reading, since the pH naturally fluctuates during the day and night especially if plants are in the tank; this is termed the diurnal cycle. It is possible the pH may lower continuously over time, and in that case with livebearers I would certainly take steps to buffer the water and keep the pH above 7. Dolomite or even crushed oral in the filter would achieve this at much less cost.
Hope this helps a bit.