It is very interesting to me that the OP hasn't responded to any of the discussion points in this post... :?
As has been previously stated, in order to make an informed recommendation between using three Group 24 batteries or a pair of 6V GC batteries, we need to know a lot more information about the the usage scenario; What is the typical current draw? What is the expected max current draw? Have you done a daily power budget? How much solar are you considering adding? Do you plan to run the generator daily in addition to the solar? Why do you want to use AGM batteries? How will they be charged?
Given the limited information provided, and considering the fact that the OP said "the electric loads are modest" and wants a solution that provides "the most amp-hours at the best price", the leading candidate is a pair of reasonably priced 6V FLA GC2 batt's... such as the ones that are available from Costco or Sam's Club. With the tiniest bit of effort given to installation and maintenance, these will provide good capacity (200+ A/h) and offer the lowest dollar cost per Amp-hour delivered over their service life.
If the OP frequently boondocks, or has a high power inverter and/or a high current charger, or a mounting location with limited access/ventilation, or an unlimited $ budget, etc, etc...I might change my recommendation.
But for now I'm gonna say; (2) 6V batteries is "better" than (3) 12v batteries.
-Mark
