Part 91 Icing Regulations.
a.
IFR. No pilot may fly an airplane under IFR into known or forecast moderate icing conditions unless one or more of the following apply:
(1)
The aircraft has ice protection provisions that meet the requirements in 14 CFR part 135, appendix A, paragraph 34 of Special Federal Aviation Regulation (SFAR) No. 23.
(2)
The aircraft has ice protection provisions that meet the requirements for transport category airplane type certification.
(3)
The aircraft has functioning deicing or anti-icing equipment protecting each propeller, windshield, wing, stabilizing surface, control surface, airspeed instrument, altimeter, rate of climb instrument, and flight attitude instrument system.
b.
VFR. No pilot may fly an airplane under VFR into known light or moderate icing conditions unless one or more of the following apply:
(1)
The aircraft has ice protection provisions that meet the requirements in SFAR 23 paragraph 34 (same as part 135, appendix A, paragraph 34, and § 23.1419 at Amendment 23-14).
(2)
The aircraft has ice protection provisions that meet the requirements for transport category airplane type certification.
(3)
The aircraft has functioning deicing or anti-icing equipment protecting each propeller, windshield, wing, stabilizing surface vertical tail, control surface flap, aileron, elevator, airspeed instrument, altimeter, rate of climb instrument, and flight attitude instrument system.




To answer the OP's question, Can I legally fly into a cloud with an OAT of -3?

Based on the AC 91-74A, YES, but only if the icing wasn't forecasted or known to be moderate (AIRMET) and you filed IFR. If you filed VFR, well, you're in violation as soon as you flew into the clouds.

But, as some have mentioned, and the author of the book I first quoted suggested, according to many FAA and NTSB reports, the FAA has drawn a very fine line between KNOWN and FORECASTED ICING.

2) Can I takeoff with an icing airmet surface to 10000. (ceiling off say 7000 with no precipitation in the forecast if I stay VFR)
a) if I do and it starts to precip, am I now open to scrutiny if something would happen?
(icing airmet and visible moisture) If you filed IFR, NO. You can't fly into an area of known or forecasted icing. VFR, yes, if you obey cloud clearance requirements for the specific class of airspace.

3) I see pireps from c-172's (assumed not certified for Flight into Icing) reporting rime icing with freezing OAT. Are they reporting that they are in violation?

If they were filed VFR, yes. IFR, maybe (was there forecasted or known reports of icing).

DISCLAIMER ... this is the best I could come up with. I'm sure there are others that are smarter than me or have better gouge. The above references came from the AC No: 91-74A circular.