I agree, the gap between the three isn't big before Luffy learnt advanced CoA.
I believe the monster trio came out of the time skip all High tiers, with Luffy being High High Tier with G4, Zoro and Sanji being Low High Tiers.
Sanji hasn't treated any Veteran characters as a threat, hell even likes of King isn't a threat to Sanji. But according to some people even with RS he is still a low high tier, or even Veteran level at that. Do these people even believe what they're saying?
As for how I scale characters in One Piece, you can see them on my Signature.