onesojourner:
I am using 3 boards and 3 batteries.
If you look at the board pictured in the original post, you can see the trace connecting the input "-" to the output "OUT-". If you wire the outputs in series with a common ground at "-" two of the "OUT+" pins will be shorted to ground. The charger may tolerate this, since they current limit by design, but shorting batteries that have any charge on them is not going to end well.