I don't remember many details of the plot, but one brother needed to become a very powerful wizard in a very short time. He basically stepped out of time to a pocket universe where he studied for many years, then came back with all the abilities he needed. I think each book of the trilogy focused on a different brother. They were all fairy short books of ~200 pages. I read them around 1983 so they may have been published as early as 1977. Any ideas?
Perhaps this is John Morressy's Iron Angel trilogy. The books are Ironbrand, Graymantle, and King's Bane. The books were originally published in the 50s, but were re-published in 1980, 1981, and 1982 (respectively). They are all between 200 and 300 pages.
The plots follow the three brothers (Colberane, Ordred and Staver) who are trying to reclaim their kingdom, with the help of magical swords, from an evil wizard who has usurped the throne.