The 3 liters should be pleny - use high quality Dexon or Mercon type ATF fluid. You can fill through the shifter OK. I would wait until the trans is installed to fill it. It you don't wait, you will spill fluid when you tilt the trans to install it. Then you have to lay in it while getting the trans installed.
I usually just unscrew the top pipe filler plug and stick a 6 foot piece of garden hose in it. I put a catch pan under the trans to catch the overflow. Then I put a funnel on the other end and sit down on the doorsill and pour away. When the trans is full, it will start dripping fluid out into the catch pan. Put the top filler plug back in, tighten it down good and you are done. The trans will have the right amount of fluid, no guessing or having the wrong amount.
