On Wednesday, Phillip Lim embroidered a sweater with a “Laverne & Shirley”-style PL monogram. Then, last night, Zac Posen trotted out a top emblazoned with his own moniker. The third designer to get in on the act? None other than Ralph Lauren, who studded the pocket of a red plaid shirt with a rhinestone RL. Three makes a trend; monogrammers should be doing a booming business come fall.