ABCD – parallelogram, AD = 12 cm, AB = 3 cm, DE and CF – bisector. Find EF.

Length AD = 12 is given for understanding the process – that it is greater than AB, accordingly, we dare to assume that E lies on BC, F lies on AD.
The bisectors of the angles of the parallelogram cut off on the sides that they intersect, the segments equal to the smaller sides, therefore EC = СD
FD = СD, we get a parallelogram (and another rhombus) EСDF, i.e. EF = 3
Answer: EF = 3