Any lead acid battery solution will not need a BMS. Pretty much any charge controller or AOI will accommodate lead acid batteries.
Three in series will work but 6 in series will net a terminal voltage of 96V. Maybe you meant two parallel strings of three batteries?
If you go with the AGM solution, watch out for excessively deep discharge cycles which will reduce battery cycle life. Most folks recommend discharge no deeper than 50%. That recommendation applies to all lead acid batteries. With lithium, the recommendation goes to 80% maximum discharge. As a result, there is a fair amount of capacity remaining with lead acid batteries that is "untouchable". You can use it if absolutely necessary, but doing so will shorten your battery life.