Young couples want space i.e. need to go for bigger room. If you have been to ID show rooms, 3 room is terrifyingly small. The minute you put a wardrobe in, your room is cut by half. Nowadays house are smaller, why go for even smaller that has a bigger price tag.
4 room has always been the preferred size for young couples to start their first house. Especially those who intend to have children and/or take care of their parents. Those who buy 3 room usually intend to flip it over after MOP.
I applied for 3 room TPY as well but dropped out. Mainly due to reason, space and no desire to sell after MOP. The only appeal is the central location but after site visit, I don't think it's really worth it. The nearest NTUC is a distance away and the road will be congested aka loud and dusty. If you're taking public, it's really not as accessible compared to red/green line in my opinion.