Betacraft (recommended) MultiMC (recommended) Vanilla Launcher
Betacraft (recommended). Follow the instructions on Betacraft's website (link) here to install Betacraft. Then, go to the server browser and scroll down until you see The Legend of Beta 1.3. Double click and Betacraft will automatically install all the game files for you and launch the game. When asked for a version number, all listed will work, however the recommended version is 1.3-xxxx. What's the difference? All b1.3 versions are identical except for singleplayer fixes and will function the same on the server. 1.3-xxxx is recommended over 1.3_01 simply because it doesn't have the unnecessary "_01" in the version number, which always shows in the top left corner while playing versions of Minecraft from before the full release.