I believe that the Lutron Caseta support 3-way and work with HomeKit. You would place the real switch on the load wallplate and then link to the remote dimmer which is a battery operated thing but can be mounted in a wallplate. I would prefer to have a "real"s switch as the slave/dummy instead of battery one because it shows the led's with dim level but I am not sure if Caseta can do this. Check out this site and click 3-way on right for video
http://www.casetawireless.com/pages/support.aspx
http://www.casetawireless.com/Documents/0301710 - 3-way.pdf
I wouldn't follow it literally hooking the wires together like they said until you determine a bit more about how the travellers work in your 3-way and know which end is the load end (top or bottom of stairs).
This seems to be an issue with Smart Bulbs because most homes use wall plates for our lighting. Phillips has the most ugly, non-standard looking switches to control their Hue bulbs and that is a big reason why I am not seriously looking at them. Not everyone in my family wants to pull out their iPhones to start turning on/off lights.
You also mention Amazon Echo. I wish it was available officially in Canada. Universal Devices just added support for their ISY994i and released a beta - many are saying great things about it and you can turn on/off lights via voice. If this was available in Canada I would definitely get one.
http://forum.universal-devices.com/topic/17861-amazon-echo-connected-home-feature-beta/