Per the discussion below, unless you have international travel roaming plan enabled for the 6S+, not sure how that phone with a Verizon SIM will be able to receive text messages from the US while you are in Europe. The 6S+ has to be on a mobile network to receive texts. If it's on WiFi then iMessage would work along with other apps like Skype, Hangouts, Messenger, etc.
So am I basically out of luck on being able to receive texts/calls on my Verizon number abroad if I put the 3 SIM in my X and my Verizon SIM in my 6S+ unless I turn the cell service on?
I do have "travel pass" which charges $10 a day (which is what i've been doing so far) and my phone works identical to how it would in the US - I have my US number and can make/receive calls from it and send SMS text to non-iMessage/iPhone users - the phone just picks up cell service from Vodaphone or O2 or another cell service that is most powerful wherever I am.
The X obviously has this feature, does the 6S+?
What it seems like I'll end up doing is putting the 3 SIM in my X, put the Verizon SIM in my 6S+, keep cellular off and turn cellular on once every couple of weeks, pay the $10 and make sure I haven't missed any important calls/texts for those who can't reach me on iMessage
Yes, you can use the Verizon TravelPass on the 6+ - it goes with the device that has your SIM. The SIM will connect and will receive calls and texts so long as you have voice roaming turned on (settings / cellular / cellular data options / roaming / voice only).
So, Verizon, in addition to the $10/day TravelPass (which, as you know, charges only when you use Verizon services that day), also has a $25/month and a $40/month travel plan that allows access to calls and texts. They charge for each ($1.79/minute and per $0.50 per message sent and $0.05 per message received charges for the $25 plan, 100 minutes included and unlimited received messages and 100 sent messages included with the $40 plan, plus 100 MB of data - which isn't a lot.) However, there is also "pay as you go" pricing that allows you to receive calls at that same price and message costs are also the same as the $25 plan, just no included data, and data is EXPENSIVE, so make sure that mobile data roaming is turned off.) However, you may as well keep TravelPass. It seems that you have this figured out well.
See
https://forums.imore.com/e?link=htt...services/international-travel/&token=K-dSBIT6 for all of Verizon's international options and costs (I am not 100% sure about Eastern Europe, but, other than that, all of Europe I have been to - UK, Spain, France, Monaco, Italy, Croatia, Greece, Hungary, Austria, Germany, Netherlands, and Czechia - are in the 140 countries with monthly pricing.)
Also, Verizon offers an app in the App Store called Verizon Messages+ that will allow you to receive messages over WiFi when you don't have data connectivity. You can put that on the iPhone X and receive messages using the app with that as well; it will stay synced with the 6+ so you can use the X for the odd SMS or MMS messages you receive that aren't iMessages.
https://itunes.apple.com/us/app/message/id621469412?mt=8&at=10l3Vy