sms 2 to 1218 is the best and most accurate to check the roaming data balance.
judging by the history of TPG/Simba, the 3gb bonus will eventually become permanent.
if you wish to switch plan, do it on your day 25th day to reduce damage as once you change plan, the new cycle will based on the day the plan is changed + the remaining days of the old plan will be forfeited. to check which day are you into your plan, log in via the simba website.
those currently on $10 100GB plan but rarely go out of singapore and don't think you will use much data at overseas, you might wish to just stick with your plan. given your 1gb free roaming data, any additional 1gb cost $4. comparing this with you pay extra $2 every month for months before you utlise the 5gb roaming data.