Why do you feel you should be talked out of sending a text?
Maybe you need to send a text for you? To almost give yourself permission to move on?
If I had left a loved one out on a line for 3 weeks and got a text, I would reply, either way. I would not be able to let someone just hang for so long without knowing if I am coming back or even if I am alive or not. That would eat me up. That said, 3 weeks can go by without me even realizing it had been that long and a text to remind me would help.
But, that's just me. For me, an emotional text wouldn't go over well so if you are going to send one, I would take all emotions out of it and be as direct to the point as possible. Again, though, I can only speak for myself here.
But, I say do what is best for you. Not for him but for you. Your needs and your mental health and wellbeing should come first to you. Regardless of what anyone else is going through. If that makes sense.
I hope it's ok that I replied to this. I think that you need to give yourself permission to reach out and then, give yourself permission to move on. It is not fair for someone to keep another dangling on a line for 3 weeks without any word. PTSD or not. You know?