Depending on the construction of the image, it may be necessary to split apart the components of the image.
Use the select tool (F1)
and click on the image. If there is a single bounding box, it is a composite image and has to be split manually. Use the ungroup tool
to determine if you can revert the image as it was constructed.
When you’ve clicked on the ungroup tool, the bounding box should change from a single outline to as many as five, three for the circles, two for the lines. If it does not, it may be necessary to split the paths manually.
As an additional test, select the image, click Path, Break Apart (Shift Control K). If you’re lucky, you’ll get the five bounding boxes. If not, manually it is. In my testing, I was unable to fail to get five bounding boxes.
If you get the five paths, jump to the last paragraph in this answer.
If necessary to manually split the paths, return to the node tool (F2)
and select a “dot” (node) at the end of any line. Use the fourth tool on the node tool bar (break path at selected node) after selecting the node. This will split off the segments selected.
Use the Break Apart feature again and you’ll see that you have an additional bounding box.
Once you have split off the circles, you would select each one in turn and use the Fill and Stroke feature (Shift Control F) to add fill in your desired color.