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)
Some people argue:
TFA: Record breaking numbers
RO: successful, but much lower than the 1st.
TLJ: better than the 2nd but not as good as the 1st.
TROS: ??
The road to final installment of the ST has been quite different from the OT and PT. Many people have argued, that TLJ feels more like the end of a trilogy than a middle act. Given the waning interest in Star Wars in general, my guess is, that TROS will end up near RO levels, which was also a December release. The fact that TROS is the conclusion to the Skywalker saga might draw in more people, but given that interest is at an all time low, I think TROS just breaking the $1B mark is not at all an unlikely scenario.