Note that the level 3 modifier is already implemented in the qwerty us-intl layout too (first page, post #5).

Except for ? and / being real level 3 and hence triggered using the yellow arrow modifier, all other yellow characters on the keyboard are triggered using Shift, as on a full-size qwerty keyboard. Same as what Mosen described for qwertz.

Nonetheless, all keys have level 3 and level 4 characters, as shown in the picture in post #5. For further details, the config file is in plain text and characters are easy to recognize from it. For instance, you'll see that I prioritized dead keys on the `~ key for level 1 and 2, and their non-dead alternatives for levels 3 and 4.