It had to happen this way, but should have happened differently...
This is neither here nor there. MK3 is what it is, a paid beta test of UMK3, with an incomplete roster. UMK3 was essentially being developed the entire time MK3 was out, and they had every intention of including Scorpion in the game from the beginning. Whether it was intentional or not, the big leap from MK3 to UMK3 allowed for massive gameplay tweaks, enough of which made them completely different games. MK3 turned MK into something that was fast paced, and could be taken seriously as a fighting game, until people discovered Sub-zero's iceclone. This is one of the most broken moves in fighting game history, and it was rightfully corrected in the game's successor.
Playing vs the AI is incredibly limited, and if you want to the beat the regular cast, simply jump back, and allow them to run towards you, then roundhouse them, every single time. Beating the bosses is also easy because they stop blocking chain combos after a couple hits. The game's tier list starts with Sub-zero Jax and Kabal being God tier, and then usable characters in Sindel, Robot Smoke, Kano, Nightwolf and Kung Lao.
Your main goal is to find people to play against in this game, but my advice would be, get the feel of the run button, autocombos, and the new cast, then move onto UMK3 quickly, and learn all the new gameplay tweaks in that.
To learn more about this game, visit www.ultimatemk.com/forum and receive tips from professional Mortal Kombat players.