Centimeters, not inches.

3 cm is 1.18 inches and 2cm is about 3/4 of an inch.

I'm going with honed granite and we're going with thicker counter. I think the skinnier one is more suitable for a bathroom.

The thickness of the c-top often doesn't matter except in cases where there is a cantilevered overhang. What kind of edge are you considering? There is no rule that indicates 2cm is appropriate for bathrooms and 3cm is for kitchens.

Here's a quick reference:
View A shows a cross-section of a c-top over a cabinet. The edge is refered to as a double-laminate bull-nose. It "caps" the top of the cabs thus making the tickness of the actual slab irrelevant since the edge gives the illusion of a very thick slab.
View B shows a similar cross-section but a simplified ogee edge. Without the "cap" it is much easier to see just how thick (or thin) the slab really is.
View C shows an overhang of the c-top. The thickness of the granite will vary depending on how far over the granite will hang past the cabinet unsupported. If memory serves me I believe 2cm will be ok for up to 6" and 3cm up to 12" (but don't quote me on that- my resources are at home).
Often times, a KD will push for 3cm when, in the case of view A, it is completely unneccessary and resultes in a larger expense. I have found this to be particularly true when the KD is sourcing the granite through a "friend" or "business acquaintance".

{{!gwi}}

We wanted a simple flat edge. Would 3cm be best in that situation?

First, it would be very uncommon (in my KD experience) to do a squared edge without a "cap" on it. The reason is because the upper edge of most cabs aren't all that pretty. That and there's usually some distance between the upper portion of the cab/drawer/door (see another one of my visual aids below). There could be a number of reasons why you might go with a squared edge that I can't see without seeing your cabs, door style and that kind of thing.

Note: Typically, a squared edge of 2cm granite would work well for the tops of tables, or case pieces, like a dresser.

In my 10+ years of KD I have never had a squared edge 2cm slab installed over cabs without some sort of "lip" or cap (as with view A abover).

{{!gwi}}

I thought I posted a reply already! Oh well, I didn't realize that the flat fronted counter tops looked like that. In that case, is a 2 cm sufficient - would there be other reasons to have the granite thicker if it has an edge like this? Thanks for the info!

I have a 3cm squared edge countertop and it looks great. I don't notice anything unattractive about the cabinet top at all.