Yes, that’s the problem. The brightness_pct parameter should be passed as data to the light.turn_on service call. This should work:
sequence:
- service: light.turn_on
data:
brightness_pct: 30
entity_id: light.hue_white_lamp_3, light.hue_filament_bulb_1
If you setup the automation using the front-end, you shouldn’t have to deal with these details.