Please reason step by step, and put your final answer within \boxed{}.
Find all functions \( f: \mathbf{R} \rightarrow \mathbf{R} \) such that for all \( x, y, z \in \mathbf{R} \), the inequality \( f(x+y) + f(y+z) + f(z+x) \geq 3 f(x + 2y + 3z) \) is satisfied.
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