Gain = 1+R2/R1
If R3 = R4 = R5,
Vout =Gain*( (Vin1+Vin2+Vin3)/3)
R1||R2 = R3||R4||R5 for minimum offset error due input bias current
For different ratio of input signals … well it gets really complex. Checkout the first link in the link section if you really want to find out.
Example setup
for Vout = Vin1+Vin2+Vin3
R1=24k
R2 = 12k
R3 = R4 = R5 = 24k
R3||R4||R5 = R1||R2 = 8k
