Flying to SG and trip to Bintan/Batam thrn back to SG is possible?

There are actually better travel related websites out there. This site is more expat related. Also, your query is better suited to be asked in the Singapore Forum. Personally, as long as you have a ticket for departing Singapore then probably I would guess it would be fine.

Singapore are very strict, especially Indonesian and Malaysian for various reasons, main one being many working illegally and import of contraband
If your planning to stay for a short while in Singapore, you will need proof of funds and a local address (if asked) ie friends address or hotel etc
You'll maybe need your onward travel documents from tanahmerah or vivocity also
Go on to the Singapore immigration website as arrival cards will soon stop and/or complete online
Would it not be cheaper and easier to travel to batam - bintan-singapore- batam airport?  Nongsa ferry is not far from batam airport to bintan but you'll need to confirm

but I would love to visit SG first then take fery to Bintan and back to SG and stay there again for 3 days and take flight back to Jkt as I am bases in Jkt, do you think it will safe?

I think that probably if you can fly to Singapore then it shouldn't be too much of a problem to visit Bintan for few days and then return to Singapore. As Gwmeath suggests, keep plenty of records with you just in case immigration give you a hard time. Hopefully the number of caronavirus cases will not expand hugely leading to much stricter immigration procedures.

thank you sooo much for all the advices! hope you have a joyful life xoxo