First thing is your vector is a float4 not a float3. It has rgba so red, green, blue and alpha. Those are 4 channels. The white top output node is the float 4 vector. You can either break that, or just use the separate channels that are below it.
You cannot condense the vec4 into a single float. If the other interaction is a vec3 then just use a vec3 in place of the vec4 or pass it through a component mask so the amount of inputs are the same.