The syphilis test can take up to 12 weeks to turn positive after someone is infected. Syphilis is really easy to transmit. It’s very possible that you got syphilis from him when you had sex 3 weeks ago. By getting the shot, you will get rid of any incubating syphilis. This will prevent you from developing symptoms and also prevent you from transmitting syphilis to any of your other sex partners. It’s great that your partner told you and that you're both getting checked out and taking care of your health.