A "3/4 bed" is an abbreviated way of saying a three-fourths bed, or a bed that is 48 inches wide and 75 inches long, according to About's quilting expert. The bed is three-fourths of the width of a full-size bed.
Although three-fourths-size beds are not manufactured in contemporary times, they were common in the past, and many vintage beds are this size. The size works well in small spaces when a bed larger than a twin is desired. Because vintage beds were often handmade, measurements tend to vary by a few inches. All measurements need to be checked before ordering custom bedding or mattresses to ensure that the ordered product fits the specific three-fourths bed.