Yes , i think you should change your flight to next morning , let me explain as i am from singapore.

you arrive at T3, 5.45pm by the time you clear immigration its maybe 6:00pm.
You need to walk to the shuttle bus station,i dont know wether T3 has it , if T3 doesnt have it you need to go to T1 or T2 and maybe it will be 6:20.

the journey takes about 10 minutes

maybe you want to eat something before leaving Singapore ?? that will take time too,so i really think you should change your flight to next morning