OK, brief course on "random" numbers:

Computers produce pseudo-random numbers (PRNs) using a pseudo-random number generator (PRNG) function. The output (i.e. one PRN) of a PRNG is determined by the "seed," a number that is given to the PRNG as input. The first time a given program produces a PRN, the PRNG must be given an intial seed - one of the common choices is to use the system time, as this results in a different set of results each time the program is executed (unless you know how to freeze your system clock, but that's beside the point). Subsequent PRNs are generated using the prior PRN as the seed, unless the PRNG is reseeded, which can be done at any time of the programmer's choice.

As a consequence, if the same seed is given to a specified PRNG, it will produce precisely the same series of results. In Civ 3, the PRNG is seeded at the beginning of a game, and is never reseeded. The current seed is saved in the .SAV. Therefore, any time you reload and perform the exact same series of inputs, the results will be exactly the same. As a matter of fact, if you so chose, you could start one game and play it out, recording all the input you gave. If you then went back to the 4000 BC autosave and played the game again, using exactly the same input, the results would be exactly the same, up to and including the modern age!

Note - one thing in this is speculative, I do not know for absolute certain that the PRNG is never reseeded... but I have gone through the Ancient era and into the Middle Ages like this, as a test, and was able to confirm that the PRNG was not reseeded even at the era switch. Thus I conclude that the overwhelming probability is that the PRNG is never reseeded.