Combine several field as a new field say my custom content type has 3 different fields: max, min, mean. What I want to achieve is group them into a new field say statistics so the new statistics field will have all the values in fields max, min, mean.

In terms of display, I now have several walk around solutions. 


in node--examplecontent.html.twig template:

statistics:{{ node.field_max }}/{{ node.field_min }}/{{
node.field_min }} 

but the problems with this is that "statistics" is hard code so it
can't benefit from the translation module if I want to build a
multi-lang site.
or I could define a dummy field "statistics", and hide it in the
form display so user wont need to enter the value of "statistics"
when create new content of the content type. then in
node--examplecontent.html.twig template: 

{{ node.field_statistics.fieldDefinition.label }}:{{ node.field_max
}}/{{ node.field_min }}/{{ node.field_min }} 

this will allow me to translate to label of field statistics to have
it dispaly as a different language.


I wonder if there are some better ways to achieve it, thank you.
I think there are several ways to go about this, but with your current field setup and familiarity with template you can use the trans functionality in Twig (see https://www.drupal.org/node/2047135).

{% trans %}
  statistics: {{ node.field_max }}/{{ node.field_min }}/{{
node.field_min }}
{% endtrans %}


Other ways of doing things:


hook_entity_extra_field_info()
Display data via views (see nizz0k's answer)
Translate in PHP in the template preprocess function
Create a new field type that stores, min, max, and computes statistics via a Typed Data method.
Similarly to the above, add a typed data data type plugin to calculate statistics.