Q: Why are my group chats sending as text messages and not imessages?

Send a group message with your iPhone, iPad, or iPod touch - Apple Support

You left out a key component of that group text your having issue with you didn't state if all are iphone users or not.

If they are, then someone have imessage turned off.

If they aren't all iphone user then someone doesn't have MMS enabled or provisioned by their carrier.

As I stated earlier, someone may have imessage turned off on their phone.

I see the issue on the other two phones, as you stated you have no issue with another group imessage text.