It's not difficult if you have the time & the right tools. And, yes, an alignment must be performed afterwards. You can borrow a ball-joint puller tool set from many auto parts stores for free. They just charge your credit card number to insure you return the tools, then cancel the charge afterwards.
If you have to replace three joints, then it's advisable to replace ALL the ball joints. The others are probably near the end of their life anyway, and this way you only need to get one alignment. Make sure you replace both ends of the tie rods, too.