The solution proposed by cellphone (sorry cellphone) isn't good and have some clear mistakes.
It's better to increase a number of unity resistors, e.g. 18:2:18 or 36:4:36 (more elements = more precise matching). Matching pattern for example
DACCAACCAACCA|ACCAACCAACCAD
DACCAACBBCAACCAB|BACCAACBBCAACCAD
DACCAACCAACCA|ACCAACCAACCAD
Don't forget about thermoelectrical effect at the resistor contacts. Number of resistors (A,B or C) must be even, and for two neibor resistors AA,BB,CC the direction of current flow must be opposite.
Good luck.