Again, I'm not sure if there's a simpler way, but here goes:
See attached schematic.
-V0=(V1-V2)/3 + V3/2
Rearrange equation to get:
V0 = -(1/6)(2V1 - 2V2 + 3V3)
Opamps U1, U2, and U3 will output 2V1, 2V2, and 3V3.
Opamp U4 outputs -2V2.
Opamp U5 along with resistors R9, R10, R11, R12, and R13 are a summing amplifer which outputs the sum 2V1-2V2 +3V3.
Opamp U6 multiplies by -(1/6).