Simple 3D effect like this in Inkscape

That is a linear gradient. To create it in Inkscape 0.91:

1. Select the object that you want this applied to.
2. On the Object menu, click Fill and Stroke.
3. On the Fill tab, click the Linear Gradient button (it's near the top of the Fill and Stroke pane.
4. (Optional) In the Name column, change the name of the gradient to something easy to remember.
   Gradients can be re-used across objects, so a good name makes it easier to manage.
5. At the bottom of the Fill and Stroke pane click the Edit button.
   A blue line will appear on the object, with a square on one side and a circle on the other. This line represents the direction of the gradient.
6. Move the nodes on the gradient so that it is in the correct direction.
   Tip: You can use snapping to help with this, just like any other nodes in Inkscape.
7. (To create the effect in the image the question) Click the Insert Stop button at the toolbar in the top of the Inkscape window.
   A new node will be added to the gradient.
8. Click each node and change it's color and opacity to the desired values.
   To create the effect in the image in the question, change the first and last nodes to a green color and the middle node to something lighter green color.

See:

• Gradients (Inscape User Manual)
• Where has the gradient editor gone? (Inkscape forums) (Gradients underwent a change in Inkscape 0.91, so you might see conflicting information out there about how to edit them).