How to create a multi-source citation

  or  

To add a multi-source citation (see How to create a citation), you can do one of the following:

   1.   Add a citation in the document:

   1.1.   After filling citations in the document, you can add any of them on the References tab, in the Citations & Bibliography group, click the Insert Citation button:

   1.2.   Choose the citation from the Insert Citation drop-down list:

   1.3.   Select the added citation and just choose any other citation from the Insert Citation drop-down list.

   2.   Add a field in the document:

   2.1.   On the Insert tab, in the Text group, click the Quick Parts button and then select Fields...:

In the Field dialog box, in the left side, select the Citation field in the Field names list:

Type the following:

<Tag> \m <Tag 2>

Then, if it is necessary, type:

\m <Tag 3>

Notes:

1. You can see and change Tag name of the citation in the Edit Source dialog box:
   
2. Instead of adding the field from the ribbon, click Ctrl+F9 to insert the curly brackets, then enter the field text between them:
   

   Where A1, A2, A3 and A5 are the tag names of the citations,

   \m is a switch that adds another source to the same citation.

3. Word does not support the concatenation of reference sequences in the way such as [1-3, 5].

See also this tip in French: Comment créer une citation multi-source.