Any consumer purpose open-end credit secured by the consumer's dwelling will fall under the HELOC rules. See the Commentary to 226.5b(1).
If the loan is an open-end credit for a consumer purpose then it will be subject to applicable rules found in Subpart B of Reg. Z for the specific credit (home secured or non home secured).
Your scenario #3 would not be subject to Reg Z because it is for a business purpose.
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The opinions expressed are mine and they are not to be taken as legal advice.