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It is the normal predicament about placing batteries in parallel. To do it properly, you need 3 ammeters.
If you only have one ammeter then you must contrive an easy way to break 3 connections so you can place the ammeter to read current through each battery string. (There is a slight problem, namely, by inserting the ammeter you add a certain amount of resistance in that battery string.)
As for measuring voltage, to do it properly you need 6 voltmeters. If you only have one voltmeter then you'll need to place the meter across each battery, to get an idea of its condition. This requires a lot of motion on your part but it's hard to avoid.
You have 4 nodes to measure to compute 6 differential values or simply the V/2 error in 3 nodes.
This tells if each one of two is imbalanced and not each pair, Generally cell matching is 1% of 1.5V for new batteries, but since Vmax-Vmin is around 1.0Volt or 11.5 to 12.5 with some load for near 0 to 100% SOC, the 1% imbalance is equivalent to 8% SoC imbalance (or 1.0/12)
If you put in current sensing on each pair connection, then you can compare each pair for ESR of the pair, which is also an indication of aging and SoC. Again ESR imbalance should be well matched perhaps to 10%.
With good connections, perhaps the current can be monitored using the mid-strap voltage drop in millivolts after calibration with an ammeter.