for 180s i'd suggest grabbing blunt. take off and keep your shoulders square while flexing your abs, and grab your tail as you keep your legs locked so your toes are still pointing at the landing. then just hold it as long as you can while you let your body slowly do the 180, and when you release your grab your feet will come naturally down to switch and you can stomp it
for shifty 3s i just carve in like normal but a little more straight up and do the first 180 as normal but as soon as i get to 180 i rewind that 180 with my legs (so spin left, then immediately "undo" what you just did with your feet) and hold it as long as is appropriate while you let your body keep spinning like normal, then when you unlock your legs they'll follow your body/head through the remainder of the spin really quick
hope that helps