I haven't brewed acacia but i can't see any reason why it should loose potency after only three days. Are you sure? Perhaps you ate something heavy before the trip and it slowed it down? I've had rue and mimosa last in the fridge for a month or two before and never loose potency. Each trip at the same dose can be so different, so there is that as well. But maybe you drank some of the sediment the first time? It can be pretty active, and after a few days can really drop out to the bottom of the brew.
You can freeze it if you want. I guess it depends on how long you want it to last. I used to brew up kilos of caapi vine and a few hundred grams of rue and just drink it when I wanted for a month or two...I would then re-boil it for a few minutes every couple weeks just to make sure it stays good, and to evap off any alcohol from fermentation. Eventually I run out of caapi and end up extracting harmalas from what is left of the rue. You can freeze it if you want, and it would probably last years if you did, but if its only a couple weeks then I wouldn't bother.
side note: Technically what your doing is not "ayahuasca"... it has to have the amazonian vine to be considered that. But thats not to say it isn't an awesome alternative! ;]
<Ringworm>hehehe, it's all fun and games till someone loses an "I"