If you just want a leather case (pocket type) that protects the TT3 well and clips to your belt, I would recommend looking no further than PalmOne itself -- get the Palm Leather Belt Case for the Palm V series. Personally, for my TT2, I use the T/T2 "Scuba" case by Palm, but it doesn't have a clip. However, when I want a case with a clip, I just use the Palm V case. It has a semi-hard front and back, the sides are soft leather, and it has a velcro flap close on top. The belt clip is metal and it is attached well to the case by some sort of metal plate that is supposed to protect the screen as well.
Here's a pic:
You can get it on eBay or Amazon for practically nothing (like $3 or $4 -- you'll pay more for shipping). I bought mine on eBay for my M500, and used it with my TT2 until I found the "Scuba" case which I like better.
Good luck! Finding a case for your Palm is like finding a case for your camera -- preferences, preferences, preferences.....
As for the rest of the replies lamenting about SD card slots, I can agree somewhat. However, it does prompt the response of, "Geesh, folks, how many SD cards do you need for one Palm?" My TT2 is only 32mb onboard, and I have one SD card that I use for MP3, MMPlayer movies, data backup, and even about a dozen favorite family photos! It's a 512mb card, and I have a couple of CD's worth of music files, one two-hour movie, a couple of Disney movies, about 1/2 dozen SpongeBob episodes, etc. I guess if you're buying dedicated game cards, a Wifi card, a Bluetooth card, etc., that could pose a problem, but if you already have one card in your Palm slot, that leaves the two slots in the case for other cards...that's three. I've owned notebook computers that have less storage on them that that, and less external hardware!
I think it's funny that we all buy Palms because they're small, useful, etc., and then what do we do? Buy a bunch of extra cards, clip-on modems, GPS devices, charge/sync cables, etc. I think we'd be better off just buying the smallest Dell notebook (which usually includes a custom case!). Please don't read this wrong -- I'm as guilty as anyone else in this, but not with regard to SD cards.