My first question would be ..... Are you shooting from a solid rest?? Then I have to explain the gun you have in your hand is not, and was not, originally a target arm.
If your only 3" off at 25 yards with a fixed sight SA, LEAVE IT ALONE.
You cannot turn the barrel in small enough increments to correct for 3" without going 3 - 6 inches the other way. You can file the rear sight channel and perhaps open it on one side enough to move point of impact 3 inches. Remember though, at 25 yards, the front sight covers about 5 inches.
Don't loose sight (nice pun too) of the intention the gun was originally intended to group in "minute of man" at around 70 yards.
Coffinmaker
PS: The most accepted method to adjust point of impact for windage is to turn the barrel a smidge. When you remove metal from the most expensive part, and screw it up, it stays screwed up