Low TC resistors for current measurement
One option is Vishay's bulk metal foil resistors. 

They have many package configurations available, but the VCS series looks good for current sensing applications.


2 ppm / C TCR typical
0.5% initial tolerance
4-terminal configuration
Much more stable in value after ESD/EOS events, compared to other types.


Pricing and availability will probably drive you crazy. I found a 1 Ohm VCS1625 part at Digikey for $16 each. For 3 A you probably need to go to 0.1 or 0.05 Ohm due to the resistor power limit.