If all three are about the same size, try calling the clinic you took your other rats to, let them know you dont have money for a visit and see if they will give you anitbiotics based on the other two's weights.
Try calling around seeing if you can make payments.
Apply for CareCredit, it is a medical line of credit which you can use for you and your family, as well as your pets.
Rats in general are extremely prone to upper respiratory infection. If you can already hear that your rat is having issues breathing, it is pretty bad. They get pneumonia really easily from having upper respiratory infections. Are you seperating him from the other two? It is very likly they will get sick as well if you still have them together. Many clinics will assist you in signing up for a line of credit to help pay the bill, or even try calling around to see if one will take payments. My boyfriend and I took in a rescue rat who had pneumonia, you do not want to see your little guy go though that. It is absolutely terrible. We took him to emergency vet and they pumped him full of medication for both the infection and pain before putting him into an oxygen chamber, but unfortunatley he did not make it. That cost about $850, which is a lot of money, I know.
If you can not get him the medicine he needs, he will suffer horribly. Having him put down (as sad as it is, it is better than sufficating) should be less than $100.