Why make them so similar? Harder to tell them apart that way.
Siobhan, Gianna, and Shinwu.
Or Sven, Jose, and Chaim.
Jerry, George and Cosmo.
Hee hee hee.
Rock, Ima, and Hardplace.
Some baseball trios:
Willie, Mickey and Duke
Joe, Johnny and Frank (first names of Tinker, Evers and Chance)
Warren, Johnny, and Pray
I think it would be kinda fun to choose two good names, and one awful name, just to fuck one of them up.
Emily, Rose, and Spinach.
Charles, Eric, and Biscuit.
Portia, Isabella, and Thud.
Tom, Ben, and Farquar-Colmondeley.
Beat me to it. 
Breakwind, Shart and BWAAAAHHH.
what?
Troy
Michael
Emmitt
Throat, Warbler, and Mangrove.
Larry, Darryl, Darryl.
I also really like the “two names that fit a pattern and one that doesn’t”
Rosencrantz, Guildenstern, Joe.
the week before the saint’s day of faith, hope, and love; are the ever popular
minadora, metrodora, and nymphodora! just call them all dora for short.
natalia, patricia, and dorothea. nicks could be tash, tish, and dot. (yes, do know twins named natalia and patricia who go by tash and tish.)
for boys you could go for the ever popular… peter, paul, and marion! thomas, richard, and henry could be fun.
sean, john, and jan…mmmm,…karen, katherine, and katrina.
Megaera, Tisiphone, and Alecto.
Alpha, Beta and Gamma. (girls names only, I think.)
ETA: Henceforth to be known as “The Particle Sisters.”
Parsley, Sage and Rosemary
(because Who knows where the thyme goes? ) 
FWIW, I know a Chinese-Australian guy named Alpha. He says economics classes were hell.
Had a soccer coach having a fourth baby, the other three were R names. He put a suggestion sheet on his door for the fourth name. My brother suggested “Rabid.”
The coach ended up not going with an R name at all.
Or co-ordinates: Xander, Yves, and Zeb. 
Faith, Hope, and Joy.
Tris
Michael, Paris and Blanket
Lois, Louis and Lillith - it may be years before she even realises her name isn’t Lillis.
Cecil, Adam, Ed