I believe there is a very literal witness within us that comes through the cleansing of our sins by 'the water'. By sins I mean the ones we still commit daily even as His children.
Ever repent of sin? To who? To the Holy Spirit (witness #1 in the verse, the spirit) or Jesus (witness #2, the blood) or is it to Witness #3, The Father? I don't know about others but I repent and ask The Father to forgive, i.e. cleanse me of sin. Matthew 6:12 Father...
and forgive us our debts,as we also have forgiven our debtors"
Not that I don't also ask Jesus or the HS also at times, as they are all one. I just feel more comfortable asking my Father to cleanse me most often. I believe John is saying The Father is the present tense witness cleansing us here in verse 7-8 by his reference to "the water":
1 John 1: 9
If we confess our sins, he [The Father] is faithful and just, so that he will forgive us our sins and will cleanse us from all unrighteousness.
I believe John has made the point that not only are we indwelled with the spirit (The Holy Spirit) and the blood (The Son) but also The Father (the water). It's why we call each other brothers/sisters, no?
1 John 2: 23 Everyone who denies the Son does not have the Father either; the one who confesses the Son has the Father also.
We literally ARE brothers because we have the same Father.
It's a great study, that of 1 John! Thanks for bringing up this topic.