Hopefully the following tips help you determine which three you should be using.
1. Determine your audiences.
Often you will find that you have multiple audiences. Starting off by identifying whom they are and what is important to them will help you focus what you will need to be sharing and thusly which app(s) will be best for which audience, based on their needs and preferences
2. How often do you need to post?
Short answer: Volume is not as important as consistency. Set a schedule for each of your chosen apps (Monday I will
blog, Tuesday I will focus on Twitter, etc)
3. It's social media, stay social.
Which ever tools you choose to use, you need to remember that the apps are not just for sharing, they are designed for engagement, connecting with others, and communicating. For every tweet you post you should have at least 3 times as many replies to others. Engaging with your audience will help you to build followers and keep others well informed.
Now the hard part, which 3 should it be?
I say 3 because the world is not all on 1 platform, and each tool has its own unique group of people using it. So if you've worked through 1-3 and you are not sure which apps to use, ask the audiences what they like to use, and start there.
Last protip for you: Determine a unique hashtag to use and use it across all of your social media tools. This will make it easy for people to discover you and your content no matter what your handle is and what app you are using.
Once you think you have your hashtag figured out, google it and see if it finds anything, look within each app for the same hashtag to make sure it is in fact not in use already.
Hope these tips help you stay connected with the world.