I have no doubt that some factions have used whatever they could get their hands on in depots, possibly a mortar shell or two. The war is just that vicious.
However its a huge leap to say they managed to collect the arty pieces and a sufficient supply for a concentrated bombardment on three different sites without anyone saying anything. The chemical weapons are the most tightly guarded depots in Syria and the Israelis are watching them like hawks.
You miss the point we have been told that it was Assad, there has been no ambiguity in the claims that Assad was responsible.