If you didn;t set the levels properly, then sadly there's very little you can do to recover it. You can raise the level, but the ratio of wanted to unwanted sound remains the same. You can try to reduce the HF but although this deadens the hiss, it takes the HF in the wanted audio away too. There are some specialist tools that can sample the noise and attempt to then subtract this, a bit like a fingerprint, but again, if the wanted sounds are very low, they're rarely much use – and pretty expensive.
I can't help with the T3 because I don't know it, but I'm sure others will advise on adjusting and monitoring your audio – recording without headphones is a bit like shooting video without a viewfinder – how do you know what the camera can see and hear?
.
