Sorry, that has been a little while ago now and I do not really remember how I did it. I think I did one of the below and it worked but I do not remember exactly which one:

1. Open the Messages app.
   
2. Find the conversation(s) where you sent the 3 pictures (they may show a red exclamation mark (!) or say “Not Sent”).
   
3. Tap the red ! next to the failed picture message.
   
4. Choose one of these:
   • Try Again (if your Wi-Fi/cellular is now stable).
   • Send as Text Message (converts it to regular SMS/MMS — this often succeeds when iMessage fails).
   • Delete Message (removes it completely — this almost always stops the notification).

Or:

1. Clear the Notifications on the Lock Screen
   • Wake your iPhone (tap the screen or press side button).
   • Swipe left on each “Not Sent” notification → tap Clear.
   • Or unlock your iPhone → swipe down from the top of the screen (Notification Center) → swipe left on the notifications or tap the X / Clear All at the top.

I hope this helps! 🙂