A.T. Robertson Commentary Romans 8:3

"For what the law could not do, in that it was weak through the flesh, God, sending his own Son in the likeness of sinful flesh and for sin, condemned sin in the flesh:" — Romans 8:3 (ASV)

That the law could not do (το αδυνατον του νομου). Literally, "the impossibility of the law" as shown in 7:7-24, either nominative absolute or accusative of general reference. No syntactical connection with the rest of the sentence.

In that (εν ω). "Wherein."

It was weak (ησθενε). Imperfect active, continued weak as already shown.

In the likeness of sinful flesh (εν ομοιωματ σαρκος αμαρτιας). For "likeness" see Php 2:7, a real man, but more than man for God's "own Son." Two genitives "of flesh of sin" (marked by sin), that is the flesh of man is, but not the flesh of Jesus.

And for sin (κα περ αμαρτιας). Condensed phrase, God sent his Son also concerning sin (our sin).

Condemned sin in the flesh (κατεκρινε την αμαρτιαν εν τη σαρκ). First aorist active indicative of κατακρινω. He condemned the sin of men and the condemnation took place in the flesh of Jesus. If the article την had been repeated before εν τη σαρκ Paul would have affirmed sin in the flesh of Jesus, but he carefully avoided that (Robertson, Grammar, p. 784).