My wife is 33+ weeks pregnant with twins . Her
I understand your concerns about placental maturity.
Grade 3 maturity of placenta is seen around this stage of pregnancy and is absolutely normal. It is expected at this stage of pregnancy.
However this does not predict the date of delivery. You can wait safely till term as any other pregnant woman to get delivery pains. Grade 3 placenta does not mean that you have to deliver faster. Only possibility in twin pregnnacy is that deliver is bit earlier than expected. You should expect to deliver anytime after 36 weeks.
Let me know if you need anymore clarification.