Ye, UsaKilleR right. But one notice - I always use all three entities. Just in case =]
Anyway... For working PK3 you need folders/files inside:
maps/yourmapname.bsp
scripts/yourmapname.arena
maps/yourmapname.shader (think its not necessary, but map w/o shaders suck ^__^)
in yourmapname.arena must be:
{
map "yourmapname"
longname "Your map text description"
type "tremulous"
}
So... Such PK3 is pretty simple, have no /textures/ and other stuff, but must work )