There are many ways how you could go about this.

The easiest way with the fewest helpers imo is to execute it every minute, check if the ac should be on or off based on your time rules and then turn it to the state you want it, if it is not in that state.

description: Turn AC on between minutes 0-37 of hours 0,3,6,9,12,15,18,21; off otherwise
trigger:
  - platform: time_pattern
    minutes: "/1"  # runs every minute

condition: []

action:
  - choose:
      - conditions:
          - condition: template
            value_template: >
              {% if now().hour in [0,3,6,9,12,15,18,21] and 0 <= now().minute <= 37 %}
                true
              {% else %}
                false
              {% endif %}
        sequence:
          - condition: state
            entity_id: switch.ac
            state: "off"
          - service: switch.turn_on
            target:
              entity_id: switch.ac
    default:
      - condition: state
        entity_id: switch.ac
        state: "on"
      - service: switch.turn_off
        target:
          entity_id: switch.ac
mode: single```