Simply put................the more "f"s there are - the finer the powder granules are, and the finer the granuales, the faster burning the powder is. On average, you can expect a 3fg powder to be 10 to 15% more powerful than 2fg for a charge of equal measure. So if your were shooting a specific load with 100gr of 2fg, you would need to drop down to 85 or 90gr of 3fg to get the same (or near same) results on target.

Triple Se7en is a notible exception to this rule though. The 2fg 777 is cut so fine that there is VERY little difference in velocity in identical measured charges ofeither grade- although I found that my Knight gets slightly better accuracy when using 3fg 777 with 350gr or heavier saboted bullets.

And another general rule the use of powder goes something like this:
1fg - cannon
2fg - long rifles .45cal & up
3fg - pistols & long rifles up to .45cal
4fg - tiny pistols & priming powder for flintlocks

that .45cal cut-off between 2fg & 3fg is pretty "general" though. A LOT of people find that their particular .50calrifles like 3fg better than 2fg. Experimentation is the only way to know for sure.