Hi mama! 36.9°C ay borderline, kaya please have your baby checked by the pediatrician today, better safe than sorry with a 3-week-old, especially with the mucus you found. Babies' temps can change quickly at that age, and a doctor can give you peace of mind right away, okay? Keep monitoring and if baby feels hotter or seems fussy/lethargic, go to the ER na. Sending you strength!