Yes, there is.
http://forums.civfanatics.com/showthread.php?t=114696
1. I would like to create a Mod or Scenario. Where do I start?
First: Pick a name. For my example, the name I'll use for a scenario/mod will be "The Spanish Inquisition". Didn't expect that, now did you?
2. Open Civ3ConquestsEdit. If you have difficulty finding it, its in the Conquests folder. In the file menu of Civ3Conquests Edit (hereafter referred to as "CivEdit"), choose "Save as..." and enter your title, SpanishInquisition.biq in this case; make sure you save it in the Conquests/Scenarios/ directory, whether it is a mod or scenario.
3. I want to have custom rules, custom players, or a custom map. How?
Before I go further, here are the ramifications of those three customizations.
-Custom Rules: This means you will be able to edit the underbelly of the game. If you want to add or edit any units, buildings, civs, governments, you need this.
-Custom Map: This is fairly self-explanatory. If you want a map with or without preplaced cities and units, you need this.
*OH NO CITY AND UNIT BUTTONS ARE GREYED OUT!*
...This is a very common question. If you want to add cities or units, press the space bar to select a civ...you can now add cities or units to your custom map.