in some case association of transistors in series improve matching when designing a current mirror with big mirroring factor.
For instance imagine you must create a current 9 time the input one. Then you connect 3 MOs as you described with the gate connected to the gate of 3 MOS connected in parallel. In that case you can see that the ideal mirroring factor is 9.
As the chrerry on the cake, you get that the area of both elements of the mirror have the same effective surface, then optimizing matching.