How often is normal?

[Rocky]

Totally normal Their average time between molting varies between sizes, teenies might molt once every few weeks, and a jumbo might wait 12-18 months between molts. You can size your hermies here: http://www.hermitcrabassociation.com/pages/sizing.htm (click on the link on the page for the actual chart)
Molting is a horomone triggered event, so it's believed that crabs can tell when other crabs are going to molt, and for safety they sync their molts so that they're up at the same time (safety in numbers) But I don't think different species can sync together.
And you're right, your crabs wouldn't molt if the conditions weren't good, so you're doing things right!

[wodesorel]

They will usually get slightly larger with every molt, but staying the same size isn't abnormal. (Nor is doubling or tripling in size in one molt!)

And yes, you have healthy crabs! Three times a year is quite good and you probably have medium to medium-large sized crabs.

Mine will molt in shifts as well. If you ever see the threads for "mass molts" around here you'll start noticing it's really common and very frustrating to have an empty tank!!

[Jemnem6]

Interesting. This explains a lot. I have a similar situation sorta.

See I had 3 hermit crabs. Then I got 4 more, and a new tank too. 1 of the original 3 buried the day before I got the new tank though, so I just left her in the old tank and put the other 6 in the new tank. And ever since then (March), the 2 original crabs have been buried. I can see one of them through the glass in the back of the tank against a wall (I use a mirror to see her). Then, some of my new guys went down too all at once. Some occasionally come up though except for one guy. And on Easter I bought another new hermit crab and she buried for a few days but is up now. Right now out of all 8 of them, only 1 ISN'T buried.

Oh and also, the molter of my original 3 came up and I put her back in the new tank and she was walking around for a few days and now is buried again with a new shell.

So I don't even know what's going down under the substrate.

[kuza]

Same for me, I've only been crabbing a couple months and I have 2 of 6 down, 2 already molted, 1 needs to molt badly but hasn't even dug down once yet. And then I have a new one. So it's very common for most if not all of them to go down soon after you get them.