Yex, it should be plenty of time baring any delays. More than enough - remember, of course, if you are able to get your bag checked through so much the better as you can stay airside and follow the signs for the connection through T5/T4 and over to TBIT.
However, if you need to collect bags at T6, you can either catch the regular landside connection bus, or just walk from T6 to Tbit going past T5 and T4 in the process (you'd turn left when existing T6 arrivals, and head up to the departures level via one of the escalators along the way - there may also be a lift/elevator inside the terminal areas - I'm assume there is
if that's easier with bag(s).
I'd estimate the landside walk from T6 -> TBIT to be 15 minutes at a regular pace but of course this can vary depending on pase.
As for it being "quiet"? Possibly In t6 yes things might be winding down a little past 8pm, but definitely TBIT will be a ZOO (and the security line will be huge). Also remember that QF will be checking in other departutes - MEL/BNE etc. The evenings at Bradley have a LOT of South Pacific/Asia flying, and a few european flights too. It's certainly a busy time there.
3 hours is definitely a good buffer - even if you're delayed an hour or so you can still do it pretty comfortably imo - at least you do not have to worry about any customs or immigration formalities, so it's pretty much akin to check in for a (US) domestic flight - but again beware that security lines can be pretty long there in the evening.