With Pz I, II and 38(t) I assume you are talking about the short-barreled 15cm sIG33 gun, there's no such thing on Pz III but the Hummel on Panzer IV with medium-length barrel 15cm howitzer.
The Grille (38(t) is said to have 15-18 rounds, depending on version. I doubt the I/II versions were able to carry that much ammo, I remember to have seen 3-4 rounds for the I. The II did not have that much stowage space either.
25/30 rounds sounds more like Panzerjäger I/II but not like the overloaded sIG33 versiosn.