Well I found these ones that are three inches

http://www.amazon.com/Natural-Unfinished-Round-Circle-Cutout/dp/B00YYCVOBE/ref=sr_1_6?ie=UTF8&qid=1462977250&sr=8-6&keywords=3%22+wood+rounds

They also have a 4" variety. Unfortunately they're only 1/8" thick which seems a bit thin IMO. You might want to plan to put them on a backing material for longevity. You could also laminate two of these together for a nice sturdy 1/4" with a bit of glue. Doing a contrasting material could be a cool opportunity though!