The London Pass is worth it if you plan on running from site to site like a headless chicken. I did buy a 1 day pass 2 years ago, and found it to be useful, but it was only cost effective if I were to go to 3 or 4 attractions in 1 day.
What I would suggest is write down what you want to see in your 3 days. Figure your first day will be a 1/2 day since you have to clear passport control, then get to your hotel, unwind, etc.
During your visit, "realistically" plot out if you can go to 1, 2, 3, or 4 places and also give yourself time to visit each place. Remember that many attractiosn close early, and you also have to take account for rest, food, and travel.
Then, compare the cost of the Travel card versus the cost of each attraction. If the cost is far better to get the travel card, then buy it.
As others posted, skip the transportation option. You can get a one or 3 day pass at any kiosk in the tube, or an Oyster card which is a transport pass good for the tube and bus