Here’s how the OT and PT did in theaters:
1st film: record breaking numbers
2nd film: successful, but much lower than the 1st.
3rd: better than the 2nd but not as good as the 1st.
And for the ST?
TFA: Record breaking numbers
TLJ: successful, but much lower than the 1st.
TROS: ?? (my guess: in between TFA and TLJ)