How to read a d1000
Roll three d10s and call them hundreds, tens and units before you roll. 4, 0 and 7 is 407. 0, 0 and 0 together is 1000, not zero. It is the same trick as percentile dice, with one more die.
With a d100
If you have a hundred-sided die, roll it with one d10. The d10 gives the hundreds (0 means none) and the d100 gives the rest: a 3 and a 58 is 358, and a 0 and a 100 is 100. On the 3D table, the d100 setting switches between two d10s and one 100-sided die, and a d1000 follows it.
Where the d1000 shows up
- Huge random tables. Fan-made lists of a thousand trinkets, rumors or wild magic surges.
- Rare events. "One chance in a thousand" is a d1000 roll of 1.
- Big numbers in one go. A population, a treasure hoard or a distance, when a d100 is too small.
Every number is equally likely
Each die is fair and each one fills its own place, so every number from 1 to 1000 comes up exactly one time in a thousand.