Re: AGM batteries
Scott, when batteries are combined into a bank it is best if they are all same size form the same production lot. Mixing batteries of different age, or batteries that have been cycled differently and are no longer in the same capacity, is not a good idea. They will equalize across each other and the weakest one will just pull down the others.
So if you have three "identical" batteries but one was used for starting and the other two for house? Don't combine them. Use 'em up, as they are, and then start fresh with one house bank of whatever capacity you want (or can find or afford) and one starting battery of the same chemistry so they can charge together on the same system.
While you are waiting for the current batteries to be used up, take a thorough look at your charging system, and plan to update, modify, improve, that as needed at the same time. Since the "little things" can literally double the life of your batteries, and batteries have gotten damned expensive, any money you spend optimizing the charging system is likely to be very well spent. Even if you optimize that now, and defer the new batteries until these are used up.