Don't get me wrong! I believe that these three things are vital to our evangelism. You see, in our society, they are some of the most widespread and even controversial problems that one might face as he goes about doing the work of the Lord (2 Timothy 4:5).
But, I think I've finally, somewhat, figured it out. The reason these three things are the front lines of the war against Satan is because... they pervert Godly institutionalism!
In the society in which we live, social science (the thoughts "concerning society and relationships among individuals within any given society") is shifting to something ungodly!
Homosexuality takes the Divine institution of marriage (cf. Genesis 2:24, Matthew 19:4-6, Ephesians 5:22-33) and turns it into nothing more than a den of fornication and idolatry. Denominationalism takes the blessing of the church, wherein is salvation and fellowship (Ephesians 1:7, Romans 8:1, Ephesians 2:19-22) and turns it into a house of false doctrine and ignorant sin. Abortion takes the original institution, that of life given by God (Psalm 127:3, Genesis 1:26), and turns it into nothing more than a blob of cancer-like cells bent on eating away at the "good life" of the parent(s).
See, they all have something in common, they take something so precious to our existence and attempt to turn the world's thinking of it into something that is a burden and even something which is upsetting to "happiness"! That's the reason that we, as members of the body, have taken so strong and outspoken a stance on these things, and rightfully so.
There are other institutions given by God which are perverted in today's world of relativism and sinfulness (worship, righteousness, understanding of our beginnings, eternity, etc.), but these three take the cake because they take what I would say are the three main things God has given to us and twist them into something for physical pleasure whether it be the pleasure of having followers (denominationalism), having the fulfillment of sexual desires (homosexuality) or having fornication without the worry of being held accountable for another human's life (abortion).
Next time you see some image scroll across your Facebook feed or some controversial statement so eloquently stated in 140 characters of less dealing with one of these topics, don't wonder why these have become the "Big Three," wonder how you can help the fight to turn these institutions back to God in our society.
I'm convinced, if we could get the world to see these three things the way our Lord sees them, there would be a lot more brethren wandering the streets of our country.