Something like this might work for you. The Inject nodes simulate the MQTT inputs

[{"id":"8d73428a.b58da","type":"inject","z":"bdd7be38.d3b55","name":"Deadband 2","topic":"topics/deadband","payload":"2","payloadType":"str","repeat":"","crontab":"","once":true,"onceDelay":0.1,"x":153.5,"y":101,"wires":[["369d10e1.a0e09"]]},{"id":"7b816efe.74b4f","type":"inject","z":"bdd7be38.d3b55","name":"Target 50","topic":"topics/target","payload":"50","payloadType":"str","repeat":"","crontab":"","once":true,"onceDelay":0.1,"x":143,"y":154,"wires":[["369d10e1.a0e09"]]},{"id":"152eb7a3.c262a","type":"inject","z":"bdd7be38.d3b55","name":"Value 20","topic":"topics/value","payload":"20","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":153,"y":271,"wires":[["369d10e1.a0e09"]]},{"id":"b924b2e3.6b4958","type":"inject","z":"bdd7be38.d3b55","name":"Value 50","topic":"topics/value","payload":"50","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":156,"y":309,"wires":[["369d10e1.a0e09"]]},{"id":"5af789a0.b6c8f8","type":"inject","z":"bdd7be38.d3b55","name":"Value 60","topic":"topics/value","payload":"60","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":157,"y":345,"wires":[["369d10e1.a0e09"]]},{"id":"369d10e1.a0e09","type":"join","z":"bdd7be38.d3b55","name":"","mode":"custom","build":"object","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":true,"timeout":"","count":"3","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":356,"y":211,"wires":[["690dac8e.d2fa24"]]},{"id":"690dac8e.d2fa24","type":"function","z":"bdd7be38.d3b55","name":"Comparator","func":"if (msg.payload[\"topics/value\"] < msg.payload[\"topics/target\"] - msg.payload[\"topics/deadband\"]/2) {\n msg.payload = \"below\"\n} else if (msg.payload[\"topics/value\"] > msg.payload[\"topics/target\"] + msg.payload[\"topics/deadband\"]/2) {\n msg.payload = \"above\"\n} else {\n msg.payload = \"inside\"\n}\nreturn msg;","outputs":1,"noerr":0,"x":513,"y":211,"wires":[["79b6233c.5b4af4"]]},{"id":"79b6233c.5b4af4","type":"debug","z":"bdd7be38.d3b55","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":541,"y":291,"wires":[]}]