Application Level MFA

I have multiple apps in my Okta Org, say App1, App2 and App3. I only want MFA on App3, not on App1 and App2. The user will always login to App1 first that should not require MFA. But when the user tries to access App3, MFA is required. We have our own login UI backed up by the Okta Java Authentication SDK. I have added a “Prompt for factor” Sign On policy on the App3. But when I try to access App3, how do I get the MFA_ENROLL status so that I can bring up the enrollment UI.