These are SPGs at br3,and are quite useless and are never used considering how their guns,the main selling points of SPGs,is on normal turreted tanks with deals more armour, more shells,machine guns,and other things that make them obsolete.I will explain each more in depth now as to why this lowering should happen.
Ho-Ni III-this SPG is at BR3 in japan,which also has the chi ha short gun and recently the chi nu,which takes the same gun,gives it a turret,2 machine guns,and double the armour value on the front (50mm),so infinitely better protection and preformance at the same BR
StuG 3 F/G-faces a similar problem to the Ho-Ni III by being a disabled version of normal tanks at that BR,no MG,no turret,much worse armour,slow,big,and you even pay around $50 for the StuG 3G to get a worse panzer 4.