I have found the solution:
In[]:=
createNetworkCorrected[]:=NetGraph[​​<|​​"Input1"->LinearLayer[3,"Input"3],​​"Input2"->LinearLayer[3,"Input"3],​​"Input3"->LinearLayer[3,"Input"3],​​"ThreeToOne"->FunctionLayer[​​Function[{v1,v2,v3},v1+2v2+3v3]​​],​​"Output"->LinearLayer[3]​​|>,​​{​​NetPort["Input1"]->"Input1",​​NetPort["Input2"]->"Input2",​​NetPort["Input3"]->"Input3",​​{"Input1","Input2","Input3"}->"ThreeToOne"->"Output"​​}​​];
In[]:=
net=NetInitialize@createNetworkCorrected[]
Out[]=
NetGraph
InputPorts
Input1:
vector(size: 3)
Input2:
vector(size: 3)
Input3:
vector(size: 3)
OutputPort
Output:
vector(size: 3)

In[]:=
net[{{1,2,3},{4,5,6},{7,8,9}}]
Out[]=
{-11.3143,0.547971,-3.11026}
BW,
Vladimir .