3.x I assume?
Depends on who is doing it and how?
For the BBEG, just handwave it. It happens.

For players: breeding generally requires a selection of base creatures to start from and careful breeding programs over generations (not something you're likely to do unless you're an elf or undead or something). The level 3 fighter bit comes from training, not genetics, so unless you want to houserule that classes can be learned in some Lamarckian fashion, you'll need a very intensive and competant training regime - which pushes the costs even higher. This will be very expensive - I don't even want to start calculating the cost per creature.

Lastly, if you want what we saw in the LOTR movies, you basically need a variant of the Clone spell that doesn't leave the extras crazy and trying to kill eachother (probably a 9th level spell at this point). Find one 3rd level Fighter with the stats you want and make an army of him.