Regarding the Trinity, it is a doctrine which we Catholics affirm to be a mystery that we can't completely understand. If the Mormon understanding seems to differ from ours I wouldn't call it a big deal since we don't completely understand it.
That being said, the Mormon scriptures explicitly teach that Father, Son and Holy Ghost are three individuals who exist as one eternal God. The Mormons seem to misuse the term Beings -- in reality when they are using the term Being they seem to be using it in a way that we would use persons. At the same time they understand there is a oneness about the three individuals which make up the Godhead. This is undeniable. Their idea of oneness would seem to me to mean that it is not incorrect for them to use the word Trinity to describe it. We can't really explain the oneness of God any better than they can.
Of course we get into a more difficult road when they start believing God had a Father, an idea which comes from what seems to be Joseph Smith's last sermon before he was martyred. It isn't King Follett, but the Sermon in the Grove that comes up with this idea. That doesn't seem to be a candidate for canonization any time soon. It is speculation which really doesn't get anyone very far since it doesn't seem to be a direct revelation from God. The logic Joseph is reported to have used in that sermon falls apart as soon as he incorrectly interprets Revelation 1:6. The interpretation is obvious nonsense which really makes the entire sermon questionable. The verse in context says:
5. And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood,
6 And hath made us kings and priests unto God and his Father; to him be glory and dominion for ever and ever. Amen.
7 Behold, he cometh with clouds; and every eye shall see him, and they also which pierced him: and all kindreds of the earth shall wail because of him. Even so, Amen.
The idea that God the Father's Father is referred to in verse 6 is simply flawed. It obviously means "hath made us kings and priests unto God and Jesus' Father" or perhaps "God, Jesus' Father. So the sermon in the grove (or at least the transcription of it) is debatable from the outset.
Which at least leaves me believing that Mormons have as much right to use the term Trinity as any other Christian. Since it is not a scriptural term Mormons tend to avoid it -- the scriptural explanation that Father, Son and Holy Ghost are one Eternal God would seem to suffice to help us understand they believe in God's essential unity.