I have a 3rd grade math gifted son, as well, and it can be challenging to find math enrichment programs that allow an 8 year old to participate or that don't assume knowledge of Algebra and Geometry. For a 3rd grader, I would look into AoPS Academy/Beast Academy for enrichment. These are much more conceptual and much less procedural than many other math programs. There are maybe 6 or 7 campuses across the country, but if you're not close enough to any of those, you can always buy the books or subscribe to the online program.
Perhaps you could get your school to offer MOEMS, CML, or Mathleague.org contests. If not, you could sign up for mathleague or CML as a homeschool participant. MOEMS has some nice problem solving books (on Amazon) that your son might enjoy looking through.
