Several things do not make sense here. First of all, the Evangelical-Free Church is very much a denomination in the same respects that "Baptist" is a denomination. Like Baptists, each church is autonomous with no governance outside of the local congregation and like some baptists, they may not like the term, but as with Baptists, the Evangelical-Free folks have "Distinctives" that make them distinctly Evangelical-Free so for this reason, I would refer to both as being a "denomination."
Being Trinitarian simply means that in this respect, they are within the realm of doctrinal orthodoxy that makes them a valid and legitimate expression of the Christian faith. Churches that are (for example) Modalist or Unitarian would be outside the scope of orthodoxy and certain denominations exist that espouse these positions (e.g., Unitarian Universalist, Jehovah Witnesses, UPCI Pentecostal, etc.)
Calvinism or Arminianism has nothing to do with whether they are denominational or non-denominational. There are some denominations that are distinctly Calvinist (Presbyterian, Dutch Reformed, Etc.) as well as those that are distinctly Arminian (Wesleyan Methodist, Christian and Missionary Alliance, Assembly of God, Etc.) Baptists and Ev-Free may be either/or depending upon the position of a particular congregation.
Finally, there is no such a thing as a truly "Non-Denominational" church. One may call themselves such but no one creates unique beliefs or doctrines that are completely different from anyone else - this includes any cult you may want to name here. A "Non-Denominational" congregation usually adopts certain denominational beliefs and practices from other denominational traditions so a non-traditional congregation may seem distinctly "Baptist" without actually claiming the Baptist name. They may also be Pentecostal (not UPCI) in origin.
