A reasonable guess is 10% less than the individual amp-hrs added together.
That said, what are you really trying to accomplish? You are attempting to put 3 used batteries in parallel to what end?
On the demand side, how many days without recharging? About how many amp-hrs per day to you anticipate using? What is your method of recharging?
As an example, in my little A-frame, the planning was like this: max of 4 nights with worst case 30 deg nights, 50 deg high during day. Max of 5 hrs run time on heater per night, none during the day, heater fan draws 4 amps. That's 20 AH per day for heater alone. In warmer weather, Fantastic Fan draws 0.8 amps on low for 19AH per day instead of heater.
Propane/CO alarm is 0.25 amps, 6 AH per day. Fridge cooling fan is 0.07 amps, or less than 2AH per day. We use mainly flashlights, minimal use of LED lights and water pump. No stereo (cutoff installed), no CPAP, no computer or phone charging while dry camping.
Worst case is 28AH per day, times 4 days/nights is 112AH. My GC-2 6V golf cart batteries are 232AH each, or 116 AH usable (stay at 50% or above). So I have enough battery to get by for the 4 nights. We don't carry a generator or solar because we are usually out doing things during the day - the camper is a place to sleep at night. If we are dry camping, battery, food, and water are all running low after the 4th day, and it's time to move on - which is less than an hour with the A-frame. In the real world, we have never dry camped for more than 3 nights in the same campsite.
The larger campers generally use more than 28AH per day, so you need to adjust your numbers.
Something to consider - if your batteries are not matched, switch them in and out individually. As long as your loads are small (5-7 amps or less), you don't lose much of the non-linear benefit of a large parallel battery bank. Use a switch like boats do to run 12V batteries separately (see photo). In my case, with 6V batteries, I just use the switch as a battery cut-off. In your case, I would use the same switch to go between your matched batteries (kept as a parallel pair) and your extra larger battery. When recharging, don't put the switch to "both" unless the batteries are reasonably equal in charge.
just my thoughts
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time