I'd triple layer...
whatever soil from the garden (dig down about a foot under the lawn is a good bet) put about 1 inch of that down in the tank (wet it so that it is saturated), then put about 1 inch of gravel on top of that (this is for the plants to get wedged in. Then, add your sand on top.
The gravel will sink into the soil. You do this, so the sand doesn't sink into the soil (as quickly, give it 5 years it'll be a brown very wild natural looking mess).
Nutrient rich soil, followed by
Unipac nordic gravel (4-6mm), followed by
Unipac fiji fine sand
Easy, kind of. This isn't 3 inches of substrate, but it's a good 2 inches.