Hello CruiseBeginner and welcome to Cruise-Chat;
a three night cruise is an excellent way to see how you like cruising; and Sapphire Princess is a wonderful ship--one of my favorites.
D114 is a good cabin in that you have passenger cabins above and below you. Being all the way forward you will have a pretty good walk to just about everything except the Promenade Deck which is two decks below you. But all that walking will just give you a healthy appetite so you can eat that much more.
The way disembarkation works is this: you pack your suitcases and put them in the hallway the night before--everything but what you are going to hand-carry. Then you collect your luggage after you leave the ship. OR, if you want to carry ALL your own luggage, you can be own of the first ones off. Unless you have an early flight out of LAX I would avoid the self-disembarkation--it is usually very chaotic. It's much nicer to go have a nice sit-down breakfast in the dining room around 7:00; then collect your things from your cabin and head to the lounge to which you have been assigned. Everyone is assigned to one of the public lounges and a time to be there. Then at the appropriate time you will be escorted off the ship.
The most important thing to pay attention to is the Princess Patter; it's a newsletter placed in your cabin each night that tells you in detail what to expect the next day. Read it cover to cover with a marking pen in hand.
Any more questions, please just ask. I live in L.A. and have been on Sapphire Princess several times and would love to help make you a cruise convert.