.The rate varies a lot when you make idle descents. But, for 3 deg slope, If you want to calculate if you are making 3 deg or not, try this, using GS:
300 kt aprox 1700 fpm
250 kt aprox 1500 fpm
200 kt aprox 1000 fpm
wrong, 3°is 5%, so divide your GS by 2
300kt. 1500fpm
250kt. 1250fpm
200kt. 1000fpm