If No. 3s follow the same change history as .223 cal No. 1s (and I have to assume they do) then your barrel will have a 1:12" twist, meaning it is best suited for lighter bullets up to 52gr, maybe 55gr. More recent No. 1s switched to a 1:9" twist to allow heavier bullets.
I'm currently having great luck with Montana Gold 55gr bullets on top of 24-25gr of Benchmark. Still zeroing in on the exact load.
-- Sam