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As the title says. 3 in the super heavy detatchnents for +3 cp. Is this by itself a legal list?

Yes.

Time to get Technical:

Open Play: Yes. Any models you put on the table make a legal list. Furthermore, this list is Battle-Forged as you have all the models in legal Detachments.
Narrative Play: Yes. Any models you put on the table make a legal list. Furthermore, this list is Battle-Forged as you have all the models in legal Detachments.
Matched Play: Yes. This is a Battle-Forged Army where all detachments share one Keyword and/or are Unaligned.

Legal? Yes.
Fun? Debateable.

Any good?
Ask Unit.