I second the notion for the tylenol. When my oldest was 6-7 months, we flew from NY to AR (7 hours total and 2 flights) and found it really helped soothe him. I also carried pretzel stick when are good to suck on instead of filling his belly with juice. As great as juice is to help them swallow and pop their ears, it fills them up quick and with the motion of the plane and god forbid any turblance, 'what goes down, must come up'! If you little girl is weaned of a passi, great, but if not, that will also help a lot with her ears. You can also try using ear wax (the kind for swimming). The wax will block out a lot of the noice from take off and landing, as well as help with any pressure issues.
Also as much as you would like to take a huge diaper bag with everything in it, don't. You should only pack enough diapers for the trip...if only 3 hours maybe 5-6 at the most, and just the necessary's.
Also if you want to get creative, get a little cheap tape player and record her a few of her favorite songs, maybe with your voice saying a couple of things in between them. Is this her first REAL trip away from you? If so it may help her stay calm being confined to a small seat, especially if you mom is holding her in her lap.
A big safey thing to consider is take a picture of your little girl before you leave the house that SAME morning and print out a couple of copies, Maybe even half a dozen. Keep one with you, and then give the rest to your mom. Make sure they are in an available spot at all times. God forbid if anything happens in the airport, it is advised to have a current picture of your little girl (especially if taken that day-you have her wearing the same clothes and everything) so that the picture may be passed around easily and quickly for everyone to be on the look out. If you have the picture available immediately, some airports can scan the image and put it up on the teleprompters as an Amber Alert. Another suggestion (and yes it sounds rediculas but it works) is to dress you little mobile darlin is flamboyant clothing such as neon green, bright yellow, something eye catching. With a toddler, she is probably going to get fussy at some point and will try to run away. This will help you spot her more easily if she escapes.