The context of Revelation 1 shapes how we read of God and His son in the rest of the book. (and impacts how we read of God and His son in the rest of the bible)
The Revelation of Jesus Christ, which God gave him to show to his bond-servants... 1:1
The one who overcomes, I will make him a pillar in the temple of my God 3:12
I will write on him the name of my God, and the name of the city of my God, the new Jerusalem, which comes down out of heaven from my God, and my new name 3:12
The one who overcomes, I will grant to him to sit with me on my throne, as I also overcame and sat with my Father on His throne 3:21
I saw in the right hand of Him who sat on the throne a scroll... 5:1
Stop weeping; behold, the Lion that is from the tribe of Judah, the Root of David, has overcome so as to be able to open the scroll and its seven seals.
6 And I saw ... a Lamb standing, as if slaughtered v5-6
What we see quite clearly (and presented very persistently) is a fitting differentiation between God and His son Jesus, the Lamb of God. The risen Jesus, the Lamb, the firstborn of many brothers, still has the same God we do.
With this plain understanding, we can grasp the meaning of other verses that may seem confusing when read alone.
In the passage in question, 22:3 has as the primary subject God (and the Lamb), but the subject is God and the 'His' referred to.
The passage closes with the focus on God as the subject providing the light. Jesus is mentioned in passing through this passage so the 'His' cannot refer to him.
So the passage must be read in light of other writings by the same author showing that God and the Lamb are not the same being - God is not the Lamb and therefore, the Lamb is not God.
Some may use John 10:30 to suggest that Jesus and the Father are one. Using proper exegetical process we must not assume this means they are one ‘substance’ or some other fanciful interpretation. To understand what Jesus meant we search other texts to find the answer.
Jesus prayed to his Father regarding the disciples,
...so that they may be one as we are one. John 17:11
And repeated again v22
Now we know what John meant in 10:30 - to subscribe to another meaning than being one in purpose is to ignore the inspired text interpretation of itself - as God intended.