This hymn was written by Puritan William Barton and is based on 1 John 5:7
You can download the hymn sheet here:
Three Witnesses There Are Above – Full Score 
I have played three stanza so the first stanza can be repeated.
Lyrics: William Barton (From 1 John 5:7)
Music: William Croft
Three witnesses there are above,
And all these three are one:
The Father, Son and Sacred Dove,
One Deity alone.
The Living Father sent the son,
Who by the Father lives:
And unto them that ask of Him
The Holy Ghost he gives.