About the 3-laps, the best shroomspot is in the long straight just after the 2 turbo-arrows (2nd and 3rd) ! The problem about the 2nd long straight is, the next corner is a hairpin, and I think you have to stop the long-boost before this end, to take this hairpin correctly !
Look at this vid for the 3-laps strat :
http://neka.sakura.ne.jp/08_koopa2.mpg !

With arrows, it's certainly possible to do another wall-tickings (Su must use them to do a low 0'57" if I'm not wrong), but I can't help you here.

About the 1-lap, I can tell you my strat for my 17"80 : I use the 1st shroom after the 1st left corner (no wall-ticking), the 2nd at the same spot for the 3-laps, and the 3rd in the last corner (with wall-ticking). The last one is certainly the most difficult to reach perfectly ! By the way, I use this last shroom and hit the wall exactly as for the 2nd wall-ticking in Mario Circuit 1 (Extra track) ! If you see what I mean, you'll be able to do under 18"00.