Since the pattern gives 18" as the length from wrist to underarm, you know how long that would be. Figure out at what measurement from the underarm you want your sleeves to hit, and then where that would be on the original pattern. Translate that measurement to row and you know how many incs will be done at that point, then use that number for your CO and work the garter stitch cuff. Though 3" is pretty long, you may only want 2". After that, continue the incs until you have 54 sts and work without increasing until the length you want.