No it was never flat-out stated SSj3 Goku could kill Boo. However, it's very hinted upon.
Here is the Japanese panel where Vegeta states that if Goku reaches full power he should be able to obliterate Boo. Afterwards, Goku makes no attempt to correct Vegeta and say "No, he's too strong." He says something along the lines of "I've been trying to [reach full power]! But it would take me a full minute to get to full strength." I would give you exact translations from Viz and stuff, but my manga is at my other house. I'm good for scanning though.