To wire 3*12 requires 4 short interconnect cables. If you connect both load and the charger at opposite Pos/Neg, it will be a "proper" charging, i.e. balanced. (Well, almost balanced). 100A current of MW will split into more manageable 33A per battery.
AGM can support higher current than flooded before the voltage under high load drops too much. And - yes, they can be charged a little faster than flooded, even "normal" AGM like Deka. Lifeline AGM can be charged MUCH faster but with mere 500W solar this is not important, IMO.