∵ABCD是平行四边形,AB∥CD,
∴∠MEF=∠MCD,∠MFE=∠MDC,
∵∠EMF=∠CMD,
∴△MEF∽△MCD,
∴EM:MC=EF:CD,
∵AE=EF=FB,
∴EF:AB=1:3,
∵AB=CD,
∴EM:MC=1:3,
∴
EM
MC=
EM
MN+NC=
1
3,
3EM=MN+NC,
同理△NEB∽△NCD,
∴EN:NC=EB:CD=2:3.
2NC=3EM+3MN=MN+NC+3MN.
NC=4MN.
∴MN:NC=1:4.
∴EM:MN:NC=
5
3:1:4=5:3:12.
故选C.