Dog has a flea..please help.

For days my dogs have been scratching quite a bit. One of my dogs specifically more than the other two. For days I have been thoroughly checking them especially the one who's been extra itchy. Have been checking them up and down and all over, over and over again with a flash light and all and hadn't found anything. Finally today I found ONE flea on the dog who's been itching the most. Just ONE. Worst nightmare come true. Since finding it I have continuously checked and checked and checked and still find nothing else on any of them including her. How is this possible? WHERE ARE THEY? And what do I do from here? I literally already want to pull my hair out just thinking about how disgusting this is and how hard they are to get rid of. Anyone who's dealt with this please, please help me. 
UPDATE! 
I called a vet clinic and explained finding one flea on only one of my dogs. They told me where there is one flea, there are always many more. Which is pretty discouraging. I went in and they gave me nexgard to give to each dog. I also bathed them and did not see a single flea in the tub..we are currently vacuuming up and down, don't know what else to do. Feel so disgusted and bad. 

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