Give access to 3 different companies who are not in my realm.

I have a unique scenario. 
I have built an app which needs to be shared with 3 different companies who are not in my realm. These 3 different corporates have 3 unique emails (one for each). Each email is a distribution list in these firms which is also a notification email in my app. An activity triggers the notification and sends an email to the right distribution list. Users from this firm which receives email need to access my app which is in my corporate realm. 

Steps taken so far:
- I have created 3 users which represent the above mentioned 3 email addresses. 

Problems faced:
- When out of realm users click on the application link, it brings them to my corporate realm which is SAML authenticated. 

How can I set up my app so that out of realm users can access my app without any issues. Thnaks