Hello
Im very new, in fact havn’t hit buy on the amazon yet. But I have a BUNCH of 3 way switched in my house, it is much more expenisve to wire these (currently i have a few running off my alarm system i plan to move everything to my smarthings)
Im sure its possible to just buy the Main and put a blank plate over the other switch? I read that motion is no the best but is controlling them with door contact reliable?
(Welcome! 
I’ve moved this to projects so you can get individualized responses based on your own needs and setup.)
As far as using sensors to trigger the light, sure, that should work fine as long as it meets the needs of your household. It isn’t really any different then opening the refrigerator and having the light come on. And with SmartThings you can have the light only come on at certain times of day or when other conditions are met. The contact sensors are if anything a little more reliable than the motion sensors, so that’s not an issue.
thank you for the move still trying to find my way around.
By the way, if you do want a physical switch in some places, there are also battery operated switches and buttons that work well with SmartThings and are certainly easier than rewiring the auxiliary position. ( you still have to have one wired master switch to control the load, but then you can add as many battery operated devices as you want wherever you want to act as auxiliaries.) See the following list:
thank you the link does not work, I can see the thread.
I know one person figure out to to make smarthing talk with the alaram.com do you know if it can use the door sensors from that sys or not?
The link should work fine, just click on the title of the thread in the excerpt.
Or use this one:
FAQ: Full list of buttons and remotes confirmed to work with SmartThings
Why not buy the Lutron Caseta. They have a video on how to setup a three way. https://www.youtube.com/watch?v=KsxI0RKdQuo
I have one and it works perfectly.
In a typical 3 way setup, you’ll have the line coming into one box, the load coming out of the other box, and a pair of travelers in between. If this is your setup, it would be easy to use one of the travelers to “jump” either the line or load from one box to the other, put a switch there, then put a blank plate over the unused switch box. But…it all depends on how your home is wired. Best thing to do is get a multimeter and get positive identification for each of the wires coming into each box and go from there. Your specific configuration will dictate what you can or can’t do, and the best way to do it.
In the US, I don’t know of any code requiring three way switches except for stairways. That’s left up to the individual homeowners. And even for stairways, there’s an exception for sensor technology:
NEC 210.70(2)©
“Exception to (a), (b), and ©: In hallways, stairways, and outdoor entrances, remote, central, or automatic control of lighting shall be permitted.”
The one exception is attics, where some local jurisdictions do require a physical on/off switch at the entrance to the attic for the lights there (although it can still be a battery operated switch, at least in most places) But other than that I think it’s just considered a convenience issue, not a safety one. As long as the lights come on when someone is at the top of the stairway, the US code does not require that there be a physical switch there. ( again, except for attics in some jurisdictions.)
Personally, I think it’s a good idea, but options like the Lutron pico’s or other battery powered devices that would still work even if the hub were not operating, including contact or motion sensors, seem fine to me.
this is how the one light i have done so far was done with the travler, but im really just trying to save cost in areas where 2 are not needed, some of them i can almost touch both switches as the same time lol
I just did the same thing on a switch pair in my garage - replace one of the switches with a GE Zwave switch, selected one of the two travelers to connect to the load, and abandoned the other traveler (capping it at both ends). Works great. In my case (and probably yours too), the COMMON terminal on the traditional switches are key. The COMMON on one switch was always hot and the COMMON on the other switch goes to the load. The other terminals are connected to the travelers.
thank you all i think one light i will do this and the rest ill get the add on