In this post dorkbotpdx.org show us a couple of different ways that an external antenna can be added to the Raspberry Pi 3.
It’s not clear why an external antenna is not an option for the Pi 3, there are a number of possible reasons and I don’t presume to know the Raspberry Pi Foundation’s exact rationale. It could be to improve the out-of-box experience, an integrated antenna “just works” and there’s no potential for the antenna connector to be broken plugging in an antenna. It could be to reduce complexity, the unit with an integrated antenna is compact and self-contained.
External antenna modifications for the Raspberry Pi 3 – [Link]