Yeah，you are right. The ik1 and ik2 failed. But that's not because of the wrong constraints different from ik3. The first and second chains are in the same plane, so it just need two constraints(x and z), while the third chain works in space and it should be constrained in x, y and z. Also, their bases are different.
At last I figured out the reason of ik-failure: I didn't set accurate position and orientation of chains in building model. It seems that building an correct model of a complex mechanism is of vital importance and should be carefully checked.
Now things got all right.
Thank you for helping!
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