Ya... DDepends which Master you are going to take, if it's MBA then you need at least 3-5 years experience other than GMAT.
If it's other master, then GMAT itself should be able to make it...
NTU and NUS are comparable, SMU is lower ranking.
...
MBA requires both GMAT and 2+ years of working experience.
SMU graduates are popular (not always, if your English is not good) than NTU/NUS finance graduates,but very expensive.
At university level, NUS'rank > NUT's rank > SMU's rank.