Well if you pick three glasses out of the 6, each one worth 1/6 probability, you'd have a 50% chance (3/6) of glass B being there (being they are randomly shuffled.)

And if you pick them 1 by 1, you'd have a 1/6 probability at first, then a 1/5, and then a 1/4, which is a 37/60 probability (61.6%.)

So if my math and logic are not wrong, it is better to pick them up 1 by 1!