Is it still a requirement that for travel to Singapore it's necessary to submit an online arrival form no more than 3 days before actual arrival?

I believe so, here, https://eservices.ica.gov.sg/sgarrivalcard/

 

But in case you don't it appears not the end of the world, from FAQ's;

 

"Travellers are strongly encouraged to submit their SG Arrival Card before arriving in Singapore, as those who have not done so will be required to submit on arrival before they can proceed for immigration clearance. This will delay their immigration clearance unnecessarily."

Yes, an arrival card is required. I beleive it can be done just before going through immigration but no more than 3 days in advance. We went a few months ago and it was a breeze - they use electronic gates to enter/exit. Just place passport on scanner and look at the camera (may have to do fingerprints but can't remember). Takes no more than 10 seconds.

They then send an email welcoming you to Singapore with your permitted length of stay and a link in case you need to extend stay. Good system.

https://www.ica.gov.sg/enter-transit-depart/entering-singapore/sg-arrival-card