Who was Jesus talking to when he was praying in the garden of Gethsemane?
He was providing us with an example of how to pray as men...because he was here to live like one of us. He did the same thing with the Lord's prayer.
What do you mean that you don't limit yourself to those three?
God can manifest himself however he wants. As Jesus he is a man. As the Holy Spirit he is a spirit. As God he is a full fledged deity. All of these are still God and all can exist at the same time. God can exist in whatever form or capacity he chooses. He was also a Pillar of Flame for the Israelites at night back in the Old Testament. Was he not God then?
In fact, virtually all Christians believe that God is One, including Trinitarians.
Because the Bible indicates that there are three. If it said that there are two...or four instead...we'd be going by that.
Nowhere in the Bible does it indicate that there are three different 'essences' or 'people'. The problem is a semantic one too. The very concept of 3 different people would mean 3 different Gods. You cannot have three different 'essences' and call them the same entity. If Jesus is known by a separate name from God and operates 'outside of him', then they are two separate 'essences'...or else it's another deity. Linguistically there's no way to reconcile this. If you can divide God into any sort of pieces or people, it's not the same God. We don't do that with people...
We can look at a section of John 14.
"Jesus said to him, Have I been with you so long, and you still do not know me, Philip? Whoever has seen me has seen the Father. How can you say, Show us the Father?" - John 14:9 ESV
Or perhaps Colossians:
"He is the image of the invisible God, the firstborn of all creation. For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authoritiesall things were created through him and for him." - Colossians 1:15-16 ESV
That section is clearly talking about Jesus being the creator, even though the Trinitarian perspective is one of God being the creator not Jesus.
"For to us a child is born, to us a son is given; and the government shall be upon his shoulder, and his name shall be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace." - Isaiah 9:6 ESV
That passage references Jesus as being "Mighty God" and "Everlasting Father" in the same breath.
There are several spurious passages in the New Testament that refer to an inherent threeness, but those have been under dispute for a long time and don't seem to mesh with the rest of the texts.
In a nutshell, God is omnipresent and can manifest himself how he pleases. It's a form of Sabellianism and is contrary to the Nicene Creed...and is hence unorthodox. I'm your friendly neighbourhood heretic, yes.