You need to determine if the phone is truly dead or just not displaying an image. The iPhone 6S is known to have backlight circuit issues. I don't understand what you mean when you say you didn't have it for 3 months. Also, what did the repair shops try. The more information you give us the better.
Here's what I would try for now:
- Use a genuine Apple Lightning connector
- Use a genuine Apple charging brick and let it charge overnight
- Disconnect the Lightning connector and then re-connect it.
- Does an iTunes enabled computer recognize your phone?
- If it does then shine a bright flashlight on the screen. Do you see a dim image?
If you have access to a USB Ammeter, you could use that to determine if the phone is really drawing current when it's charging. You could have a dead battery. Apple has a recall program for the iPhone 6S experiencing unexpected shutdowns. I would check out the site to see if your phone is eligible for a free replacement.
If you see a dim image, then you probably have a defective backlight circuit on the logic board. If you don't see an image, then you most likely have a completely dead logic board (or battery, see above). Anything logic board related will require a micro-soldering repair and is not DIY.