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Hi revlis. Which town is better as a base depends on your preferences. Mürren and Wengen are both very scenic and located beautifully at high plateaus along the mountain slopes, but less convenient in terms of traveling. You’ll have to take a cable car ride down from Mürren or a train trip down from Wengen to head for other directions. Grindelwald is located a bit more centrally for some trips, larger and more touristy. Lauterbrunnen would be the most central out of the four towns you mention; it’s rather small and quiet too. Please compare traveling times with the aid of the timetable. Hotels in all towns can be found here.
Here's an overview of activities. Not all cable cars and mountain trains will be open in early May, but there’s still more than enough to choose from. Many consider the Jungfraujoch and/or Schilthorn to be highlights.
A Swiss Pass will cover your Jungfraujoch trip up to Wengen or Grindelwald. From there, you’ll have to buy a 25% discounted ticket. A Swiss Pass doesn’t make sense for a 3 day stay though, as the shortest variant consists of 4 days. Which pass is best depends on your further plans, including how and from where you plan to enter and leave the country. These guidelines may be of help.