I used to be on Remicade, now on the biosimilar Inflectra. I had Moderna vaccines and had a pretty severe response to it for the second shot. Not an allergic reaction, just pain flu-like symptoms, fatigue, etc. But with the third dose there was no reaction.
I was sure to time the doses around two weeks before an infusion to have the Remicade as much of a trough as possible and allowing two weeks for antibody response before the next infusion.
I think you are right that this third dose is giving you a significant response. Not a bad thing in my opinion.
Jan