LMGTFY.
I’m not aware of an 80-300, but there’s a 70-300 and an 80-200.
http://en.wikipedia.org/wiki/Canon_EF_80%E2%80%93200mm_lens
Weight: 1,330g / 275g / 260g / 250g
http://en.wikipedia.org/wiki/Canon_EF_70%E2%80%93200mm_lens
Weight: 1,490g / 1,470g / 1,310g / 760g / 705g
So assuming a Canon lens it’s somewhere between a quarter of a kilo and 1.5 kilos depending on exact model.
Assuming the heaviest and adding the ~3/4Kg body weight above gives us a total of two and a quarter kilos, so the 3Kg limit tripod leaves us three quarters of a kilo spare for flash guns and so on.