Thank you for reaching out to us! The resend to non-opener's feature is still available for all third-generation templates and can be found during the scheduling process right under the section where you chose a time to schedule for later! If you are using a legacy template or have already sent the email, you would need to resend to non-openers manually.
I apologize for any confusion we may have caused by saying 3GE. 3GE stands for Third Generation Editor, which is the most recent version email campaigns we provide. The good news is you do have access to this editor. The email campaigns listed in your account as "Email" are all apart of this editor. Not only this, but also any brand new emails created will be in our Third Generation Editor as well. We have also updated our original response to show this correct name.
You are correct, at this time our resend to non-openers feature is not currently available for our custom code emails. There are two works around we suggest in the meantime. The first workaround being to manually create a list of non-openers through your reporting tab and send a copy of the email to that list. The second workaround is to create a Segment for this report, add that segment to a list. I apologize if either of these workarounds cause any inconvenience for you. Being able to have this feature on a custom coded template is a feature request we have tracked in your account.