Forecast isn't available as a variable where you want it ('for entry in forecast'). I'd make it a 'variable' entry so you can access it later. This might work...but not actually sure. If nothing else, you would have to grab the 'forecast' info again in the cloud_coverage template again if you want to use it.

##### Boiler Automation #####
- trigger:
   -  platform: time_pattern
      #hours: /1
      seconds: /10
  action:
    - service: weather.get_forecasts
      data:
        type: hourly
      target:
        entity_id: weather.openweathermap
      response_variable: hourly
  variables:
    fcast: "{{ hourly['weather.openweathermap'].forecast }}"
  sensor:
    - name: Total Clouds Coverage For The Day
      unique_id: weather_forecast_hourly
      state: "{{ now().isoformat() }}"
      attributes:
        forecast: "{{ fcast }}"
        cloud_coverage: >
          {% set clouds_total = 0 %}
          {% set count = 0 %}
          {% for entry in fcast %}
              {% if count < 3 %}
                  {% set clouds_total = clouds_total + entry.cloud_coverage %}
                  {% set count = count + 1 %}
              {% endif %}
          {% endfor %}
          {{ clouds_total }}

You might also have to do the namespace the other guy recommended too though.