My dear friends,
We're gathered together here virtually to mourn the loss of three African dwarf frogs (Hymenochirus Boettgeri), aptly named Mike. They have deceased fairly quickly after each other over the past weeks.
They left behind, apart from their bereaved and saddened owner: two African dwarf frogs (Hymenochirus Boettgeri), aptly named Mike.
Mike, Mike and Mike will be missed. Let us remember them as they were when they were alive and well.

One dead froggie is sad, but not worrying. Small animals sometimes just die for no visible reason. Three, however, in a short period of time, is a reason for concern and Googling.

Here is what I found.
Apparently, it's a deadly fungus. The symptoms match. I hope the two remaining little guys make it; in case it helps, I have added seven grams of unrefined seasalt to the tank, as recommended in the article.

I can't help worrying about the shop where I got the frogs, though. The salesman said the frogs were on special because they did a little too well breeding them, and there were 8000 of them in stock at the time. I hope they are being treated...

I'm also worried about the fungus wiping out large amounts of the world frog population. But I can do even less about that than I can do about the frogs in the shop.
I might send an e-mail to the shop... maybe it helps.