What you’re looking for is... template variables a.k.a TVs
You create your 3 TVs (in your case RichText TVs), assign them to the templates you want to use it with, then add the tags to your template : [*box1*], [*box2*], [*box3*] or whatever you want to name them.
Then when a client edits a document using that template he’ll have 3 fields on top of the content field (if you don’t want to use it and have it displayed upon editing, use the HideEditor plugin). Your client just edits the fields and voilà, your content boxes are updated
No snippet, function or php needed... welcome to the power of MODx
It’s just the basics, all you need to know to get started is in there :
http://wiki.modxcms.com/index.php/Template_Variable
You’ll get a grasp on default values, the different TV types, even @BINDINGS to tie to an external data source or script... enjoy !