....but they are vulnerable to wall fire which targets them and 3 upgrades as cats is tricky. I use cats because I want to upgrade them so they get to L4 faster as artillery. I'll take down a wall early then get my cat in there for some garrison XP.
The light cav promotion is also mighty fine
Bottom line is that, you should not need field artillery replacement, you would like a better bombard... but it's not necessary.
Do you play them much? 3 seems an excessive waste of production and they do take up a tile, they are quite lumpy.
Infantry corps with RAM... or in my case redcoat corp with RAM suits me just fine, army great. I never seem to get a tower up there but I guess I could just buy one.