A 3° descent angle in a clean CRJ200 at 210kts is very near idle thrust. If we wish to slow down after having already intercepted the glideslope, we will need to employ additional methods.
My technique:
While intercepting the glideslope from above is bad practice, APR mode will do it.
Which method is better? Should we reduce the descent rate just a little bit? We will slow down, and it might not take us too far aboive the 3° glideslope. Or should we reduce the descent rate significantly? We will slow down quickly, but might have a long ways to descend again to recapture the glideslope.
Using some general assumptions and a lot of math, I made the following chart. Descending at 210kts along a 3°, the vertical speed is about -1100fpm, if we wish to slow, we must select a smaller vertical descent rate. The X axis of the following chart is the range of descent rates we may pick from -1000fpm to 0fpm. Once slow, we must again descend and recapture the glideslope.
The left Y axis shows the number of nautical miles the entire maneuver will require. The right Y axis shows the maximum altitude deviation in feet our manuever will take us above the 3° glideslope.
It looks like a descent rate anywhere from -0 to -400fpm will keep the maneuver within 3NM. The deviation is about 400ft, which really is not too bad.
Selecting a descent rate in the -600 to -800fpm range might feel more moderate, but the manuever will take almost twice as far to complete. The distance above the glideslope will not be much improved at 300' vs 400' anyways.
Keep in mind, the real airplane doesn't change descent rates instantly like my math assumes, but selecting a -100fpm descent should be ideal.