If I have 3 batteries 100mAh 200mAh and 300mAh what the total charge in series?
Never put batteries of different capacity in series.

In your example, when you start with fully charged batteries, and your system has drawn about 100mAh, your system will likely continue to draw current.  At that point you're going to reversely charge the 100mAh battery which is not good for any battery!  Physical damage is to be expected.

You can't rely on (dis)charging only 100mAh because your 200mAh and 300mAh would get fully charged at some point anyway (trickle charging, etc).

If you were using capacitors, your voltage would not be as stable as with batteries.  when drawing a constant current from a capacitor, your voltages drop lineary, while with a battery it drops slightly over time and drops heavily near its end of charge.

Note: you can put batteries of a different capacity in series, but only do so if you are going to do that for a short period of time and within there limits - for example when you do not have enough batteries and you need to power a device to resolve a very short term situation or do a short test.