There's a few things to consider here.
Firstly some theory though. You mention watts, voltage and amperage in your question. These are all different but related. Watts are a measure of power. Voltage and amperage (or better: current) go hand-in hand, like water pressure and flow.
Now for your charger you need a supple that provides 3 volts at 2.8 amps. The voltage will be constant, the current (amps) it draws will probably depend on the amount of batteries your charging - 2.8A is the maximum.
The relation between watts (P for power), voltage (V) and current (A for amps) is simple:
$$P=V \times A$$
So for your battery charger you need to provide at least 3Vx2.8A = 8.4W of power.
Now don't assume you can 'just' use a 8.4W or larger solar panel. The charger needs a constant 3V. A solar panel's voltage depends on two things: the amount of sunlight, and the 'load' - or how much current is drawn from the panel by whatever you connect to this.
Unfortunately, this relation is not constant, and to achieve the maximum power from the panel you need some device the 'regulates' the load and then provides a constant voltage to your device. This type of devices are available 'off the shelf' as 'solar power supply' or 'solar regulator'.