This is all before the patch that changed instance levels around.

I was able to 3-box with paladin tank, druid healer, warlock dps up to ZF, which was cake once I got dispelling poly with the felhunter down. I don't remember what the lowest level I did it was but it had to be low 40s for an almost full clear(no mallet). BRD at 51-53 didn't work too well. Could easilly clear the first room and the jail area, but anything after that was trouble. ST at those levels went...OK I guess. I didn't do a full run half because I hate the place and half because my equipment was breaking at the end. Not sure if I could have handled both the dragons at those levels. I hate mara. At around 50 I went with one more, he had to leave, got completely lost later and gave up. Not sure if I could have cleared everything, but I wasn't doing too bad in the first half.

I doubt you'd get very far in TBC instances with 3 at the instance's level.