The varnewmsg bit is meant to change to on or off to send on the the next devices. The Mqtt ones work fine but the entity.landing_lamp doesn’t. And I’m not sure if I am writing it wrong. Sorry on mobile atm.
As written newMsg will allways have a topic of Landing Lamp
How could I get it to have the state? Eg. On or off?
It's hard to understand exactly what you are trying to do here. Don't worry we all had to start somewhere 
Perhaps you could share more of the flow and a description of what you want this function to do, even a sketch might help !
Looking at your function -
You don't seem to be using the incoming topics as they are commented out in the logic ?
You don't seem to be using topics on the outgoing messages either, as there are 3 MQTT OUT nodes which already have the topic set. This makes this code redundant, and you cannot define 3 values to the same variable like this anyway as it will always be the last value you set as I pointed out.
var newMsg = { topic: "zigbee2mqtt/Stairs Switch/state_l2"};
var newMsg = { topic: "zigbee2mqtt/Landing Switch/state_l2" };
var newMsg = { topic: "Landing Lamp"};
To send to the topic you define you need to leave topic blank in the MQTT node, and then only have 1 node.
You can set payload and topic within your if statments like this
var newMsg = { topic: "zigbee2mqtt/Stairs Switch/state_l2", payload: msg.payload } ;
As it stand if any of the 3 MQTT topics sends an On or Off payload, the function will stop at the first matching condition - eg if (landingstairs === 0) it will then send an ON payload to all three outgoing topics.
I'm guessing that all three will then send a new message back into the function, and after several iterations it may stop if there are no more matches ?
Regarding landinglamp there is no outgoing topic for that in the flow you shared.
