Car shifted into gear when in park...

Alright guys I'm back at it. I got a new RVMB from a member here and all went pretty smoothly. Got it put in no problems adjusted shift linkage properly. Put a new deep pan on and 7 more quarts of fluid, still shows a bit low in nuetral. Makes a bit nervous putting that much fluid in so I go out around the block and no 3rd gear. Can this be a symptom of low fluid? or should I say screw and have a local shop just rebuild the dang thing?