delete messages from messenger?

Clearly when using messenger for some time, the messages stored on the device are becoming too many.

But here it seems even more problematic. When a message is addressed to let say 3 recipients, this results in 3 separate messages on the device. To delete them, one has to delete the same message 3 times. This is definitely not so with other inreach devices, there a message with 3 recipients can be deleted once only, if any answer is here simply delete the thread is enough.

So far I could not discover any rational method of deleting messages from the messenger device.

Am I missing something?