But, please do the same calculation as i did for your A320 and your DR400 with exactly the same figures. 5000 feet to lose using the 3times rule and transpose it to the DR400 and let me see what the ROD is for both for the time/distnace covered to the destination.
A320, assume 250 knots groundspeed. DR400, assume 95 knots groundspeed. (*)
5000 feet to lose according to the 3X rule requires 15 nm in both cases. Something the A320 covers in 3.6 minutes and the DR400 in 9.5 minutes at their given speeds.
For the A320, three degree slope (or 5000 feet to lose in 3.6 minutes) leads to 1388 fpm. Entirely doable in a pressurized jet (I assume - I've never flown those).
For the DR400, three degree slope (or 5000 feet to lose in 9.5 minutes) leads to 528 fpm. Entirely doable in a non-pressurized piston single.
Now of course, if for some reason, instead of descending at 528 fpm, you start descending at 1500 fpm, you will end up at your target altitude way before the planned point. Simply because you flew about a 9 degree slope instead of 3 degrees. If you routinely use descent rates of 1500 fpm in light aircraft then you might want to think about applying a 1X rule instead of 3X. And have a permanent eardrum perforation, otherwise it won't be comfortable or healthy in the long run.
But why would you want to use an ROD of 1500 fpm in a small aircraft?
(*) A&C, sorry, the DR400 I fly is a -120 with 95 knots. I know your -180 is a lot faster.