Azure AD connect to multiple forests

Hi,

I'm trying to set up Azure AD Connect with a sync from 3 forests to 1 tenant. However, I'm struggling to add the forests in the AD Connect wizard.

I'm working with the following domains:

intranet.city.com with Netbios name "INTRANET"
intranet.pdwxcity.com with Netbios name "INTRANET"
city.com with Netbios name "CITY"


In the "Connect your directories" page, I was able to add the "parent" (company owning the tenant) domain intranet.city.com but I couldn't add the other domains. All the required ports are open. I am getting the following errors (depending on the format of the credentials; Netbios name, full domain name, UPN, etc):

The provided user was not found in the directory. Specify a valid domain account
OR
The specified forest does not exist or cannot be contacted
OR
The specified domain does not exist or cannot be contacted


Any idea what the problem could be? Is the fact that they have similar Netbios or top level domains a problem?

Please note that I am working with the hosts file for name resolution to the other domains. I don't know if this is a problem or not.

Thanks a lot for the help!