For attractions & tickets use the app Klook, you can find some good deals there. its easy and hassle free.
For Singapore, try Universal Studio, Sentosa, Marina Bay, Gardens by the bay, Orchard Road (Shopping),, Haji Lane (halal food), China town, Singapore Flyer (wheel), try the River cruise ride from Clark Quay,(optional zoo, aquarium, safari)
3 days in Singapore are enough to cover the main sights. Better give 3 days each to KL & Langkawi and the remaining to explore for Thailand