I have experience of two rtc's, the bigger of the two going from 8" to 31.5" in 14 months. I did have perfect water quality, a large tank and good quality foods and certainly from a very small initial size like yours at 3" the growth rate will be both linear and very fast. I would say around 3" per month should be expected until it's at least 18", then slowing (a little) from 18-30".
After 30" growth definitely slows down. I certainly don't think you should let yours exceed 15" maximum in your present tank and with such a small gallonage you must be extra vigilant with water changes (both volume and frequency), water quality checks and your filtration system. As far a your peacock bass goes, because the rtc growth rate shall outstrip it so much, even allowing for the 3" size advantage it has now, I would say it would need to be moved within 3 months at the most or it shall probably get eaten, with your red tail being moved to a bigger tank within 4 months. Would agree with Ethnics in terms of moving him as soon as possible, and look at his early growth rates quoted. Hope this helps.
