Is it possible to connect a simple 3-way switch to a smart switch? The model is a moes go WS-US-RF single L
Which model is it? The 3 way smart switch just replaces 1 of the 3 way switches.
This drawing is not good either way as it show the light on 24/7 
Tho looking at their diagrams, a neutral is always needed.
Right, for some reason no manufacturer that I have seen has deigned to make a wifi version of a spdt switch that does not require a neutral connection. From your drawing it seems that you are using a multi switch module. The problem with that is, that is not how a spdt switch works. It could possibly be done with some clever programming which would use two single pole switches tied together where the are always in opposition to each other, i.e. one is always closed while the other is open. Programming would have to insure a break-before-make situation to ensure both relays are never closed simultaneously. Then, if you wanted to retain local switch control, you would have to devise a way to control the relays with the local switch while not overriding the wifi control.
C’mon manufacturers, this is not rocket science. Just replace tha spst relay with a spdt relay and bring the extra connection out. The funny thing is that they are already ysing a spdt relay and not using the NC connection.