These are letter blocks that are each shaped like three different letters when viewed from three orthogonal viewpoints. The technical name for this type of thing is a 3-dimensional ambigram.
Using the customizer
There is a customizer which you can use to create your own models with words of your choosing. You can use UPPERCASE letters, numbers, a space character, or any of the special characters: $&<>*+@/\♥♠♦♣♪♫♀♂←→π.
THERE IS A NEWER IMPROVED VERSION THAT HAS A MUCH GREATER CHANCE OF MAKING VIABLE BLOCKS AND IS VASTLY BETTER THAN THIS VERSION. USE THE CUSTOMIZER THERE INSTEAD OF THE ONE IN THIS VERSION: Customizable Triple Letter Blocks Ambigram
Because of limitations in presenting letters in all three orientations this is more practical for making single letter blocks, such as for a monogram, although words or other strings of characters can be created if you're lucky enough to find compatible words.
This is because not all letter combinations will produce viable blocks that show all three letters. Some letters just aren't very compatible, such as an "L" and a "T" - one has material only on the left and bottom and the other has material only on the center and top, so no matter what the third letter is, it won't produce a single viable block unless the top letter is the T and is aligned with the right letter, and the front letter is the L. In that case 17 of the 26 letters for the third letter make viable letter blocks, but there are no other viable blocks in any of the other 11 orientations with any of the 26 letters for the third letter.
You can't tell from the preview which blocks are viable because even viable blocks look messed up in the preview, so you'll have to create your thing to examine the blocks, or render it if you're running it locally with OpenSCAD. Note that although there may be bits floating in the air which will make it look like a block isn't viable, sometimes those bits are not necessary and could be removed. Slic3r has a "split" feature which can be used to remove those bits.
Sometimes you may generate a model with all viable blocks, but the model itself isn't viable. An example of this is "GOOD LUCK DUDE", where all blocks are viable, but the middle two letters aren't resting on the base. This can easily be fixed by lowering the letters into the base, which is what I did for the STL here by moving the middle two letters 7.5% lower (into the base).
The orientation of the letters is important and most letter triplets have a best orientation. There are six permutations for placing the letters on the block faces and since the top letter can be aligned with either the front letter or the right letter, that doubles the number of ways that a block can be made with the 3 letters. These can be represented as:
FRTᶠ, FTᶠR, RFTᶠ, RTᶠF, TᶠFR, TᶠRF,
FRTʳ, FTʳR, RFTʳ, RTʳF, TʳFR, TʳRF
where F is the front face, R is the right face, Tᶠ is the top face aligned with the front, and Tʳ is the top face aligned with the right. See the GEB 12 model for an example showing all 12 permutations.
The customizer app has an option to generate six permutations at once so that you can more easily find any viable orientations and pick the best one. Try it with the top letters aligned to the front and if no good model is created, try it again with the top letters aligned to the right. If your words (or names) are several letters long the customizer may take 10 or 15 minutes to run when it's generating all six permutations.
In order to accommodate printing the letters separate from the base, the customizer produces separate STLs for each part (letter blocks, base, flat characters) as well as an STL with all parts.
This is a remix of my Customizable Dual Letter Blocks Illusion which produces viable blocks for all pairs of letters / numbers / special characters.
2019-04-08 V2.1: Added the capability to add small flat characters to the base. Also now produces separate STLs for each part (letter blocks, base, flat characters) as well as an STL with all parts.
2019-04-12 V2.3: Added option to align the top letters with the right letters instead of with the front letters, doubling the number of possible letter orientations.
I've included a few generic models which I found had viable word combinations: the "GOOD LUCK GRAD" model might make a good graduation gift, the "I♥U MOM" (or "I♥U MUM") model might make a good gift for Mother's Day (or any other day), and the "GOOD LUCK BABE" model or "GOOD LUCK DUDE" model can be used to send your sweetheart or a friend a good luck wish.
Note that there are two versions of the "I♥U MOM" and "I♥U MUM" models. To make it easier to print, MotoRider42HC created a remix of the "I♥U MOM" model by extending the back part of the U/M block so that it rests on the base. Unless you have soluble supports, you probably want to print the letter blocks separate from the base anyway so this shouldn't be an issue. But in case it is, my solution is to just lower the letter blocks 1.1 mm into the base as I've instructed to do for any floating blocks created by the customizer. Both solutions have their pros and cons, but MotoRider42HC's hand-crafting is probably better, so if you want the "I♥U MOM" model without the flat heart characters on the base, check out MotoRider42HC's remix of "I♥U MOM".
The "GEB" model is inspired by the cover art for the book "Gödel, Escher, Bach: an Eternal Golden Braid" by Douglas Hofstadter, which is where I first saw a triple letter block 40 years ago and which is the inspiration for this creation. The "GEB" letters are so compatible that all 12 permutations produce recognizable, viable blocks, although a couple of the blocks have an unnecessary floating bit which could easily be removed if you wanted to print them. See the GEB 12 model which shows all 12 permutations.
I've also included the "TRIPLE" model which is representative of a model where all three words are the same. Any word made of only the letters "BEFHIKLOPRTUWXZ" (model with all those letters also included) can be made into a model like this. The triple O and triple U are viable but by default would be floating in the air, so they're shifted down onto the base. The triple O is really funky looking so perhaps it shouldn't even be included in this group.