Trifocals are bi focal reading glasses with a third lens section. You need trifocal rather than bifocal glasses if you have trouble seeing your computer screen, book or needlepoint (hey, people still do it) within an arm's length.
Bifocal eye glasses and trifocal eyeglasses simply correct more than one vision problem simultaneously, e.g. macular degeneration, presbyopia, myopia, hyperopia. Bifocals primarily correct presbyopia. Trifocals just add eyesight insurance for people whose vision doesn't improve with bifocal reading glasses. If you think you need trifocals instead of bifocals, consult your optometrist.