Hello Ronald: After a raspberry cane bears fruit (one or two times, depending on variety) that cane will die. At any one time, the plant will have fruiting canes and vegetative canes. You can expect the vegetative canes to bear fruit next season and then it is their turn to die while the plant produces additional vegetative canes for the coming year. It is very likely that the yellowing canes are the ones that had fruit this season. This may be the first time you have seen this as your plants are now just 3 years old.
Sincerely, Richard