Voltage Mixer with 3 Resistors
Using superposition you can show that:

(R1||R2||R3)/R1 = 1/2

and 

(R1||R2||R3)/R2 = 1/6

So if we set R1||R2||R3 = 10K (a more-or-less arbitrary choice)

R1 = 20K 

R2 = 60K 

R3 = 1/(1/10K - 1/20K - 1/60K) = 30K

Let's simulate it: