You need all curves to be trimmed to each other. Looks like your line and 2 splines share common endpoints (good). This generates 1 domain. Your 2 corners are not sharing endpoints, this generates 2 more domains. Your line and splines need to be trimmed to your corners and it should work.
As Ricky said: edit Corner1 and choose the Trim Element 1 and Trim Element 2 options. Repeat with Corner2 This will give you a good Join with 1 domain.