If you join your values into an object, you can then use Javascript or JSONata to sum the values
JSONata Example.
$sum($$.payload.*.$number($))
[{"id":"519ff94a2e8374aa","type":"inject","z":"b9860b4b9de8c8da","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"a","payload":"true","payloadType":"bool","x":110,"y":3140,"wires":[["e337904c45b25764"]]},{"id":"e337904c45b25764","type":"join","z":"b9860b4b9de8c8da","name":"","mode":"custom","build":"object","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":true,"timeout":"","count":"3","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":250,"y":3180,"wires":[["60cab298a57210e3"]]},{"id":"96935e53f65c9842","type":"inject","z":"b9860b4b9de8c8da","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"b","payload":"false","payloadType":"bool","x":110,"y":3180,"wires":[["e337904c45b25764"]]},{"id":"e72de01a45294348","type":"inject","z":"b9860b4b9de8c8da","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"c","payload":"true","payloadType":"bool","x":110,"y":3220,"wires":[["e337904c45b25764"]]},{"id":"60cab298a57210e3","type":"change","z":"b9860b4b9de8c8da","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$sum($$.payload.*.$number($))","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":400,"y":3180,"wires":[["c896bb713eec327c"]]},{"id":"c896bb713eec327c","type":"debug","z":"b9860b4b9de8c8da","name":"debug 297","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":470,"y":3080,"wires":[]}]
[edit] if the incoming messages are objects you can still join all objects and output the sum with
$sum($$.payload.*.$number($.status))
where status
is the name of object property holding the boolean.