“I am preparing three T3 trust returns for 2018, 2019 and 2020 for an executor whose mother died in June 2018.”
"The proceeds for the sale of the house have already been distributed by the executor "
Ouch!
You also saw this discussion re late GRE at
Late filing of graduated rate estate trust designation ?
See T4013 to see what may apply to this trust.
https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/t4013/t3-trust-guide-2016.html#P65_1863