What ever route you decide the 120 watt panel isn't enough. While the rule of thumb is 100 watts per 100 aH of hose battery our experience since 2012 is 140 watts per 100 aH and even this will fall short with shade. While 3 batteries in parallel is doable it takes a bit of work to properly balance for the charge/discharge cycle. Keep jumper wires between batteries the same length and guage.
Check here for wiring
http://www.smartgauge.co.uk/batt_con.html