Is it the fact that there is a long registry? Yes, partially
There is nothing rude in a short or a long registry. A long registry just gives the givers more choices. Calling it "gift grabby" is attributing motivations for which there is no evidence; there's no obligation on you to buy more than one gift so the range of selections really isn't relevant to you at all. General advice for registries is to register for a wide range of items, from potato peelers to crock pots. When I was a student, I know I appreciated my friends who registered for lots of small items because it made it easy to put together baskets that would fit my budget.
Simply registering for items that you
wouldn't register for doesn't make the couple rude. I have to say that this is the first time I've heard someone object to there being too many inexpensive items. The usual complaint is that the HC registered for too few items and those items are all big ticket.
Some of this, I think, comes from the idea that wedding gifts are supposed to be used to set up a new couple in a new household. The nature of society has changed drastically and very few people go from their parents' home to their wedded home. I think you'll be much happier if you just regard wedding gifts as simply a gift in celebration of a happy event, rather than to meet some specific societal need that no longer exists. If you don't feel comfortable giving a gift simply in celebration of a major life event, then you probably shouldn't be attending the wedding.
Some, too, comes from the (wrong) view that registries are somehow demands. The point of a registry is to help
the guests choose appropriate gifts. A registry gives information about styles. It helps avoid duplicate gifts. It's a convenience for the guest because then they don't have to call up someone close to the HC and ask "what do they want/need?" Not every guest invited is going to have a close enough relationship
to the HC to have intimate knowledge of their likes and wants, down to the pattern on their bath towels.
Is it the fact that there are three showers? Yes because that seems over the top
Three showers, if the guest lists don't overlap aren't over the top at all. Three showers with overlapping guest lists are not polite. Even in that case, I wouldn't be overly critical of the HC because they may not be able to regulate all (or even any) of the guests lists. Over-enthusiastic shower hosts are not unusual.
That said, an invitation is not a summons and you are not obligated to attend more than one shower, if you attend any at all. Anybody commenting on your absence from one or all of the showers would be rude.
The purpose of a bridal shower is to help a couple trying to establish a household, not to help upgrade one.
When society was different, you would be correct in the purpose of a shower. The underlying situation has changed. You could argue that showers should then go away, since they aren't strictly necessary.
It feels like a gift grab because it is one.
I disagree as do others.