Dear bones, it seems to be universal that the folks here are recommending that you DO NOT grind your 3 F . When I read your question, I cringed. Definitely not safe.
Concerning 3f and 4 f as priming powder, I shoot with the international team, and everyone is now using Null-B as a priming powder. (It is equivalent to what would be 7f) very fast and reliable. As for 4F, many shooters are using 30 to 40 grains of it as a main charge in a .45 caliber pistol in 25 meter competition. For me, I find that the excessive recoil causes me to flinch. BUT, the heavier charges do result in better accuracy in a smooth bore pistol.