series battery connection with different voltages
From the title it seems you're concerned that the two three-cell packs will total 3.6 V each and the one two-cell pack will put out 2.4 V.  Connecting eight 1.2 V cells in series will give you 9.6 V.  It makes no difference that they are in different packs.  Think of the internal connections in the holders, and the wires between the holders, all being zero ohms.  The physical grouping of the cells makes no difference to the total voltage.