Your Arduino code is sending the strings in four separate messages. Best thing to do is fix the Arduino code as @colin suggested.
You could fix it in Node-red you would first join the 4 incoming messages using a timeout setting of say 1 second. You could then split the joined string. This would give you correct message parts.
e.g.
[{"id":"54c6f04fe78d91ad","type":"inject","z":"da8a6ef0b3c9a5c8","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":180,"y":3960,"wires":[["8a718d8401ddf425"]]},{"id":"8a718d8401ddf425","type":"template","z":"da8a6ef0b3c9a5c8","name":"","field":"payload","fieldType":"msg","format":"handlebars","syntax":"mustache","template":"1\n 2\n 3\n \n","output":"str","x":340,"y":3960,"wires":[["e4665a53689dd0d7"]]},{"id":"e4665a53689dd0d7","type":"split","z":"da8a6ef0b3c9a5c8","name":"simulate incoming data","splt":" ","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":530,"y":3960,"wires":[["43fffb16a6a4b95d"]]},{"id":"43fffb16a6a4b95d","type":"join","z":"da8a6ef0b3c9a5c8","name":"","mode":"custom","build":"string","property":"payload","propertyType":"msg","key":"topic","joiner":"","joinerType":"str","accumulate":false,"timeout":"1","count":"","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":230,"y":4100,"wires":[["80101fc614b1eaf8","7f53dcee207d6397"]]},{"id":"80101fc614b1eaf8","type":"split","z":"da8a6ef0b3c9a5c8","name":"","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":430,"y":4100,"wires":[["879ade3ab99a8ea8"]]},{"id":"7f53dcee207d6397","type":"debug","z":"da8a6ef0b3c9a5c8","name":"debug 230","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":630,"y":4000,"wires":[]},{"id":"879ade3ab99a8ea8","type":"switch","z":"da8a6ef0b3c9a5c8","name":"","property":"parts.index","propertyType":"msg","rules":[{"t":"eq","v":"0","vt":"num"},{"t":"eq","v":"1","vt":"num"},{"t":"eq","v":"2","vt":"num"}],"checkall":"false","repair":false,"outputs":3,"x":650,"y":4100,"wires":[["82ecd20f34c36675"],["533f818aa46c97a2"],["0aaf55c41f00fa6b"]]},{"id":"82ecd20f34c36675","type":"debug","z":"da8a6ef0b3c9a5c8","name":"debug 227","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":650,"y":4180,"wires":[]},{"id":"533f818aa46c97a2","type":"debug","z":"da8a6ef0b3c9a5c8","name":"debug 228","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":670,"y":4220,"wires":[]},{"id":"0aaf55c41f00fa6b","type":"debug","z":"da8a6ef0b3c9a5c8","name":"debug 229","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":670,"y":4260,"wires":[]}]
[editr] fixed join node error