An eight note lasts for half a count. Three are eight eighth notes in one bar of * time. When eighth notes are joined together the tails are replaced by a beam.
Here is a sol-fa exercise using eighth notes. Take them slowly and evenly until you can do them easily, then sing them along with the recorded backing. On the recording, the example is sung the first time and then a space has been left for you to sing on the repeat. Breathe at the end of each half note.