Make sure the other 2 contacts have there phone numbers set to Mobile. I don't it's going to let you send it if it's set to Home, Work, Company, etc. So just double check the phone numbers in your Group is there and that it's set under the right type.
I have a group of 8 contacts, and I test with one of my contacts that has 2 phone numbers stored: Mobile and Company. I deleted the Mobile and tried to do the group text and it only should "7 of 8 members."