i'd say it should be a lot easier if you had an aftermarket shifter, our shifter sucks especially downshifting from 5th to 3rd, everytime i've tried to do this fast, i've found myself looking for the 3rd gear gate and hitting the wall between 3rd and 5th.
i had a short shifter in my mach1, (i wish MGW had a shifter for our trannie, best short shifter ever created, take a look at the little sucker in action,
http://www.mgwltd.com/shifthandle/bullittshifts.mpg) and i could shift 5th to 3rd almost as fast as 4th to 5th, it was that good.
unfortunely,it's really hard to do a fast clean shift from 5th to 3rd with our stock shifter.
i've found it easier to shift from 5th to 4th, then 4th to 3rd
here is what i do, although not good for racing because of the time it takes, but at least i'm not hitting the wall between 3rd and 5th, besides, is fun as hell
1) push clutch
2) short rev (matching rpm)
3) shift to 4th, release clutch
4) push clutch
5) short rev (matching rpm)
6) shift to 3rd, release clutch
if you do it this way, you won't have to find the 3rd gear gate from 5th, it would go to the desired gear without any problems. from 5th to 4th you don't need to find the gear,(example: 4th to 5th or 5th to 6th) it would go straight to 4th, same for 4th to 3rd. you can skip step 2 and 5 if you don't know how to downshift matching your engine revs, but it's a recommended practice for us m6 owners, it save some clutch in the long run.
you can also save some time just skipping step 3 and 4, just push the clutch, shift to 4th then go to 3rd, release clutch.
Have fun