Mega Man X3/Bit, Byte, and Vile

Cutscene
After Mega Man X defeats two Mavericks, the following cutscene will occur.

After this cutscene, Bit and Byte will appear as bosses in the middle of stages. Typically, Bit appears during the 3rd stage and Byte during the 6th; occasionally, they may appear later. Furthermore, they will definitely reappear in a later stage if Mega Man X is defeated during a battle versus any one of them.

At the same time, three teleport hatches leading to Vile's stage will appear in three specific stages.

Bit

 * Original name: ヴァジュリーラＦＦ (Vajurīra FF)
 * Location: third stage (or later)
 * Weakness: Frost Shield (B. Buffalo's) or Triad Thunder (V. Catfish's)

If you defeat both him and Byte with the weapons stated here, they will not reappear as boss of the first final stage. A different boss will be there, instead of them.

Byte

 * Original name: マンダレーラＢＢ (Mandarēra BB)
 * Location: sixth stage (or later)
 * Weakness: Tornado Fang (T. Rhino's) or Ray Splasher (N. Tiger's)

If you defeat both him and Bit with the weapons stated here, they will not reappear as boss of the first final stage. A different boss will be there, instead of them.

Vile

 * Original name: ヴァヴァ (Vava)
 * Location: teleport hatches in three stages: B. Buffalo's, V. Catfish's and C. Crawfish's;
 * Weakness: Spinning Blade (C. Crawfish's) or Ray Splasher (N. Tiger's)

The three teleport hatches will stay in the stages even if the boss is defeated. Therefore, you can and should visit the Vile's stage after you obtained at least one of the weapons stated above. The hatches disappear if you defeat all eight Mavericks.

If you defeat Vile with the weapons stated here (at least the final blow), he will not reappear as mini-boss of the second final stage. A different boss will be there, instead of him, and you can then obtain Zero's Saber.