Extract substrings with certain length randomly from a file with Bash

I have multiple text files, and from each file I need to extract random contiguous substrings with a certain length.

For example, I need to extract Five random substrings that consist of 3 contiguous characters each, or 4 random substrings that consist of 20 characters each.

In practice, let's assume this is the content of one of the files

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so if I want Five random substrings that consist of 3 characters each, I expect an output that looks like this for example:

elc sta tec res rce

Your help would be much appreciated.