Save this in a text file like "BG CHR.asm" ..or "jriva BG Bank.asm" or something like that. I do not know how to use this with mm3 improvement exactly.
I used Odyssey to write this ASM file originally, cause i dont have any ASM files or setups for regular megaman 3/etc.
This can be expanded anytime, as many levels as you want...as many times as you want, i only did just a couple examples for now.
Here's hex version though...only compatible with mm3 improvement and if your hack is using mm3 improvement as a base
12920-12922: Level ID numbers, i did 00 (needle), 08 (doc needle), and 0C (wily 1) for placeholder
12923-1292A: CHR Bank Page numbers, in pairs of 2 bytes each. First is tile 00-7F, 2nd is tile 80-FF
First 2 pairs is Doc Needle, Second 2 pairs is Wily 1, so far
For screen numbers, set to FF if you want to disable these placeholder spots
12953 + 12957: Screen numbers to change graphics - Doc needle
12968 + 1296C: Screen numbers to change graphics - Wily 1