Then we'd need a fourth agency to coordinate between them, and don't forget the fifth agency in charge of oversight.

And don't forget to give all the money and power to the offensive agency, while keeping the oversight agency to an intern in a broom closet somewhere.

I actually approve of your plans to keep the oversight agency really minimal.

That government is best which governs least.

Sounds good to me

How about NSA just does this, per its name, and the offensive signals intelligence gets handled by the CIA?

CIA is primarily HUMINT (and intel triage, covert operations, etc) so their focus is going to be on the end points for individual targets. SIGINT is a gigantic field and a shift like this would essentially just a merger of NSA's SID into CIA while leaving NSA with just IAD.

Genuinely curious, what would be the gain in doing that?