I’m new to this and still learning. I have 72v. 1-48V and 1-24v together I thought that was the proper AH. It is 3kwh.
Okay... but where did you get that 70 Ah value? If a battery has a nominal voltage of 72 V (which this doesn't, but we'll get to that) and a nominal energy storage of 3 kWh, then it must have an amp-hour capacity of
3,000 Wh / 72 V = 41.7 Ah
because one amp flowing from a battery at one volt is one watt of power. Either the voltage, or the amp-hour capacity, or the energy capacity is incorrect.
Many people refer to the 12S modules as "48 volt", but that would be the fully charged voltage (4.0 V/cell), not the nominal voltage (somewhere between fully charged and fully discharged) of 3.75 V/cell. The GM specs for this battery (like most modern EV batteries) say that the battery, with 96 groups of cells in series, has a nominal voltage of about 360 V, so
360 V / 96 cell groups = 3.75 V/cell
... which is about the 3.8 V that you are expecting.
So the 12S3P modules (12 groups of 3 cells) have a nominal voltage of 12 cell groups * 3.75 V/cell = 45 V, and
the 6S3P modules (6 groups of 3 cells) have a nominal voltage of 6 cell groups * 3.75 V/cell = 22.5 V
Yes, they will need 48 V and 24 V to charge fully, so the charger needs to be able to deliver about 72 V if you combine those two modules in series. That part is right.
As described in a useful GM publication
2016 Chevrolet Volt Battery System, the entire first-generation Volt battery has
Usable Energy 10.2 – 11.2 kWh
Total Energy 16.0 – 17.1 kWh
Depending on where in the range your particular battery falls, and whether you are counting the entire capacity or what the Volt actually uses, that's 28.3 Ah to 47.5 Ah, and normally the full capacity is reported and the Volt battery is considered to have a 16 kWh energy capacity, so that's 44.4 Ah.
With 44.4 Ah capacity and 67.5 V nominal voltage, the combination of the two modules has an energy capacity of
44.4 Ah * 67.5 V = 3 kWh
so that energy capacity value is right, too.
But when you're charging this combination of modules, from fully discharged to fully charged, you need to deliver about 45 Ah, not 70 Ah.