How do I read the data sheet for a gate driver to determine the total output current?
3.5 amperes is the maximum instantaneous current that the device can sink or source, but is not a realistic value.  Since the device has a typical output resistance of 3 ohms, the device would have to be shorted directly to ground and attached to a 10.5-volt rail to produce the 3.5 amperes.  This would imply that the device would be dissipating 3.5 amps * 10.5 volts or 36.75 watts, which would result in a very short life.
For a more realistic idea of what each package will do, refer to the &quot;Power Dissipation Discussion&quot; on sheet 14 of the data sheet you mentioned to calculate the temperature rise of each package.  If you are switching a FET, you will have high instantaneous peak currents as you charge up the gate, but the current will drop to essentially zero between each switch.  So you will have to calculate total power from the power required for each switch and the switching frequency of your application, remembering that all of the current you supply will be flowing through the driver's output resistance.