The game doesn't use historical equipment counts, as evidenced by infantry and support equipment counts. Division equipment is balanced around relative production rates of different equipment, not a realistic count. So while an ART battalion has 3x as many guns as a real one, an INF battalion has at least 10x LESS, so the argument about ART is moot. I think keeping them at 3 width if fine, it's a significant firepower upgrade.
The issue, the REAL issue is the battlefield combat width. It is currently 80 - this makes 40 width divisions optimal. This will happen no matter what width you set it at. If you set it at 81, 27 is now optimal. 27 is the width of a standard triangular division: 3 line regiments of 3 battalions, one artillery regiment of 3 battalions. This width doesn't allow for other things to be added though - divisions usually had other things like AT, AA and tank battalions attached. Take a US infantry division - it would have 9 INF, 4 ART, 1 AA, 1 TD and 1 MARM for 35 width. A US armored division would only be 24 width (3 MARM, 3 MECH, 3 SPART, 1 TD, 1 AA) due to only having 6 line battalions instead of 9. There's no way for both widths, both of which are historically accurate, to be optimal. You have to choose. So long as combat width is a thing, you cannot have division width diversity.
I propose a combat width of 60, making 30-width optimal. This allows you to take a standard triangular division - the most common type - and slap 3 extra width onto it to be whatever: either another artillery battalion, 3 AA/AT, or another line battalion (infantry, tanks or tank destroyers) plus an AA/AT.