You just need a single set of quotes around entity id value.
{
"entity_id": "light.front_porch_dimmer_switch_level,light.main_garage_exterior_dimmer_switch_level,light.main_garage_exterior_dimmer_switch_level",
"brightness_pct":20
}
To import the below JSON click on the hamburger menu, the stacked three lines, in the top right corner of the node-red UI then Import and Clipboard. Then paste in the string below.
[{"id":"8a640f98.26ad7","type":"eztimer","z":"5eb3594f.d294b8","name":"","autoname":"dusk - dawn","suspended":false,"sendEventsOnSuspend":false,"timerType":"1","startupMessage":true,"ontype":"1","ontimesun":"dusk","ontimetod":"17:00","onproperty":"payload","onvaluetype":"json","onvalue":"{\"data\":{\"brightness_pct\":20}}","onoffset":0,"onrandomoffset":0,"offtype":"1","offtimesun":"dawn","offtimetod":"dusk","offduration":0,"offproperty":"payload","offvaluetype":"json","offvalue":"{\"service\":\"turn_off\"}","offoffset":0,"offrandomoffset":0,"mon":true,"tue":true,"wed":true,"thu":true,"fri":true,"sat":true,"sun":true,"x":246,"y":1376,"wires":[["afbea5e8.89f438"]]},{"id":"afbea5e8.89f438","type":"api-call-service","z":"5eb3594f.d294b8","name":"","service_domain":"light","service":"turn_on","data":"{\"entity_id\":\"light.front_porch_dimmer_switch_level,light.main_garage_exterior_dimmer_switch_level,light.shop_exterior_dimmer_switch_level\"}","mergecontext":"","output_location":"","output_location_type":"none","mustacheAltTags":false,"x":586,"y":1376,"wires":[[]]}]