How soon can you have an ultrasound done

Nybol • I`m 21,married been trying for baby #2 for 8 faithful months with glow. First BFP March 9 then MC On March 18.
Just fount out I was pregnant I'm now 3week and 6 days. I schedule withy doctor and she wants to see me on march 20 and do a ultrasounds. 
​How soon can you a ultrasound?