3D Touch is based on pressure sensitivity, which is probably why you have to press hard with your whole finger. See this article for information on adjusting that sensitivity: Change 3D Touch sensitivity on your iPhone - Apple Support
I did find this general article that has more technical information on 3D Touch (it's for application developers, so realize that it's not necessary information—it's just interesting): https://developer.apple.com/ios/3d-touch/