OC,
I think you shouldn't need an airstone.
There are three popular choices for "plant substrates" that I can think of:
1) Mix Laterite with the substrate of your choice (sand or gravel). Can be messy, and Laterite needs to be replaced occasionally. Also, from what I've learned, can be very messy upon introduction. (The small box should be enough for your tank -- probably twice, if I remember correctly -- and would cost about $8 US).
2) Use Seachem's Flourite exclusively as a substrate. Very popular with plant enthusiasts, and lasts much longer than Laterite. I hear it takes a lot of initial rinsing, though. Generally a brick-ish color. (cost for your tank would be about $18 US).
3) Use Eco-Complete exclusively as a substrate. Seems to have about as high a standing as Flourite among enthusiasts, but it's a newer product. Comes in a bag, like Flourite, sand, or gravel, but Eco-Complete is packed in water. No rinsing, just add to the tank. Eco-Complete is black. (Cost for your tank would be about $24 US).
Of course, you may find that plain old sand, with maybe a few root tabs if you have heavy root plants, will work just fine.
About the light, though, I'm going to have to say to be cautious, as I think you'll find that you may not really have "3 wpg" in the sense that people talk about as lighting levels.
Others may disagree, and I could be wrong, but I think the "watts per gallon" rule of thumb doesn't seem to work very well for very small tanks, possibly partially because the short bulbs act more like a point source rather than a line source.
Just as an example, I know for certain that my taller 46g bowfront tank with 60W of light is much, much "higher light" than my shorter 10 gallon with 15W, just by watching plants thrive in one, and die in the other. (under pretty much identical conditions, except for the lighting and tank dimensions).
What I'm trying to say is you may find that, even though technically you have 3wpg, you may not have good results with "high-light" plants.
That's not to say that you shouldn't look into a CO2 system, and/or a product called Flourish Excel by Seachem. Both will provide carbon for your plants, and can be used simultaneously if preferred.