If no low-speed compromises were made, what would an ideal Mach-3 wing look like?
One Mach 3 design which did not care about low speed characteristics was the Lockheed Q-12 / D-21 reconnaissance drone, which used a Marquardt RJ-43 ram jet for propulsion and had a flight Mach number of up to 3.5. It was initially air launched from the back of an A-12 / M-21 and crashed when fuel ran out. The purpose was deep penetration of hostile airspace to perform photographic reconnaissance in pre-satellite days. When later the launch aircraft was switched to the B-52, the D-21 needed an enormous booster to bring it up to its cruising speed of Mach 3.35.
This should be closest to a uncompromising Mach 3+ design; however, it drew massively on the aerodynamic work with the A-12 / SR-71, so probably the solution space was restricted from the start.

D-21 three side view (picture source). The anhedral was needed to compensate for the rolling moment of the high tail in sideslip. An even more uncompromising design would had used a smaller vertical and an equal fin on the lower side and no anhedral. The lower side fin, however, would have made the air launch from the back of the A-12 impossible.
All surfaces had sharp trailing edges. A wedge-like cross section like the tail of the X-15 only starts to make sense at speeds in excess of Mach 5 when using drag for directional stability is more effective than adding more tail surface.