WHOSE WORD IS THE WORD OF GOD?
The contextual order of Hebrews 1:3 shows that the Word of His Power is the word of the Father's power. "He (the Son) is the brightness of His glory (the Father's) and (the Son is) the exact image of His Person (the Father's), and upholds all things (the Son uphold all things) by the word of His power (the Father's)."
Notice that the context of Hebrews 1:3 starts with the Son being the brightness of the Father’s glory and that the Son is the exact image of the Father’s Person. The structure of the first two clauses of Hebrews 1:3 which begin with the Son and end with the Father points to the Son upholding all things by the word of the Father’s power in the third clause.
It is highly unlikely that Heb. 1:3 is talking about “the word of” the Son’s “Power” because no scripture in the Bible ever shows that Jesus possesses his own Divine Word (or logos) distinct from the Father. Jesus clearly stated that the "logos which you hear IS NOT MINE, BUT THE FATHER'S (John 14:24)."
Jesus said in John 10:49, “I did not speak of myself, but the Father Himself has given me a commandment as to what to say and speak.”
Wherefore, Jesus did not have his own divine brightness of glory as a distinct coequal God Person because he reflected the brightness of the Father’s glory.
Nor did Jesus ever possess his own distinct word (logos) as the scriptures prove that he spoke the Words of the Father's Power. For Jesus clearly said in John 14:24, “the word (logos) which you hears IS NOT MINE, BUT THE FATHER’S.” Therefore the Son of God could not be another coequally distinct God Person beside the only true God the Father.