For 3 degree descent, 5 x GS is a good approximation. The formula is derived from Tan (3°) = V/S ÷ GS.

Tan (3°) = .0524

.0524 = V/S ÷ GS ⇒ (.0524)GS = V/S

GS units are nm/hr, need to convert to fpm.

(.0524)GS nm/hr x hr/60min x 6000 ft/nm = (.0524)(100)GS = V/S

5.24 x GS = V/S(fpm) to be precise.