Hi Nabila,
It sounds like the female is trying to use the seed container as a nest. If your parakeet’s abdomen/belly looks enlarged, this is not normal. Birds do not get pregnant or full of eggs. They develop one egg at a time, in about 48 hours, and then lay it. There is rarely any external sign that she has an egg ready to be laid unless there is something wrong. An egg can get stuck, which would cause the female to strain and breathe heavily and if she is unable to pass the egg, she will die. Parakeets will also get tumors, and the abdomen is a common location. Often these are harmless fatty tumors, but it can still cause problems. You probably need to go ahead and take the female to the Vet.
As far as having three birds together, this is generally not a good idea, especially when you say the cage is small. It puts the female under too much pressure having 2 males in a small cage with her, and the males may end up fighting over her. Even if all three birds were the same sex, when you have an odd number of birds, 2 will usually form a bond leaving the odd bird out. The single bird will end up being picked on and bullied. You need to think about getting another cage for one of the birds.
You also mentioned you are feeding seeds, which will not provide the nutrition that they need. Please check out our feeding guide to learn about what is the best food for your birds.
Bird Food Guide
Thank you for asking Lafeber,
Brenda