2001 Mitsubishi Galant help on my transmission

i have a 01 mitsubishi galant and my transmission fluid is a little bit low. im currently searching for a transmission fluid that suits my car. i even bought a haynes repair guide. it saids that the only trans fluid i should put is diamond atf sp 3 or equivalent. now my question is, can i put Dexron III/Mercon Automatic Transmission Fluid instead of diamond atf sp3? will dexron mess up my transmission or my torque converter? because diamond can only be purchase on mitsu dealers and you know how much dealer charge you for that! im a little bit low on a budget because im still a student.