"Mobile data is disabled, this msg will be sent when mobile data is enabled" II don't have a data plan, why would I need data to send a text to two people? Never had this problem with previous Samsungs....
Your phone is probably configured to send a text message to multiple people through MMS. MMS requires data.
Check your messaging settings on your phone.
I don't have an S3, but on an S4, you would do the following:
Open Messaging > Settings > MMS, and uncheck the Group Messaging option (send a single message to multiple recipients).