My apologies to the OP - I answered the wrong question. The 3*thousands-of-feet rule gives about a 3.1% glide slope, which would help to intercept from above. The 3*distance (2.8 degrees) rule would help to intercept from below.
BOAC:
My abject apologies. My poor little pea brain is incapable of realizing the H = 3*D can be written as D = H/3. If you ARE using altitude, though, I sincerely hope you are not above mountains.
- Your humble and obedient servant