I am trying to downgrade to ios 4.3.3 but each time I try this, my iPod Touch 3g goes in a recovery loop with an all black display and I can't do anything but restoring ios5.
Is it possibile to downgrade to 4.3.3?
If you have your SHSH's saved for 4.3.3, then you should be able to get back to 4.3.3 with some help of TinyUmbrella (it also has a button to get out of recovery loop, maybe you can try that?)
If you don't have your SHSH's saved, there is no way you can get back to 4.3.3 (or any firmware version for which you didn't save your SHSH's, for that matter...)
Official download links are to the right of this page: http://thefirmwareumbrella.blogspot.com/
You cannot get back to older iOS versions because Apple no longer signs them digitally. The only way to get back further than the ones they're currently signing is using TinyUmbrella where it will sign the package for you.