Bit of a chestnut this one!
The Russian, Greek etc Trisagion seems to be directed towards the memebers of the Trinity whereas the Coptic one seems to be directed to Christ Our Lord only.
In a book I read by an Athonite monk he uses this fact as evidence that the Copts had deviated from the Apostolic faith by adding to the Trisagion.
I haven't a clue about this, it seems good to pray to our Saviour or the Holy Trinity.
However, as a Russian Orthodox who sometimes prays from the Agpeya and also as someone who longs to see unity among Orthodox Christians I would like some clarification and maybe a bit of explanation as to the differences.
In case you didn't know what we say in the EO churches is;
Holy God, Holy Mighty, Holy Immortal have mercy on us (three times)
God bless you