

I.e. is used to restate for clarification. It doesn’t really relate to the other two, and should not be used when multiple examples are listed or could be listed.
E.g. and ex. are both used to start a list of examples. They’re largely equivalent, but should not be mixed. If your organization has a style guide consult that to check which to use. If it doesn’t, check the document and/or similar documents to see if one is already in use, and continue to use that. If no prior use of either is found, e.g. is more common.
Exactly. If you’ve got a head for remembering Latin, i.e. is id est, so you can try swapping “that is” into the sentence to see if it sounds right.
E.g. is exempli gratia so you can try swapping “for example” in for the same trick.
If you forget, avoiding the abbreviations is fine in most contexts. That said, I’d be surprised if mixing them up makes any given sentence less clear.