I noticed my 3 weeks old baby has difficulty breating at night when latching. There will be lots of sound like blocked nose while she suckle as well as suckling sound. Should this be allergy? It only happen at night time.

It's normal for them to sound like they have block nose at young age as their nasal airways are very very tiny. So what you can do is to keep baby elevated. Or you can also use humidifer, provide some moisture to the air in the room so baby nose won't be this blocked and stuffy

It could be a mucus backflow. Does baby sleep in a aircon room?