Is it legal to modify GPL v3.0 content and distribute a binary of the remixed content without disclosing the modified source code?
The easiest way for you is to distribute source code and compiled code together. That way you have fulfilled all your GPL obligations and you have no requirement to do anything for anyone in the future.
The alternative is to distribute the compiled code, together with an offer valid for three years to supply the source code on request to anyone ever asking for it. So distributing without supplying the source code right now is legal, but you must supply it later when requested.