No, they are not mutually compatible. The music rest for the GO:KEYS 3 IN is not compatible with those for the GO:KEYS (GO-61K, GO-61KL), as they are specifically designed to meet the specifications of each model. When purchasing, please make sure to order the music rest that is compatible with the model you are using.
Please also see the related article "GO:KEYS 3 IN: Does the GO:KEYS 3 come with a music rest?"