Does Mapper 28 support MMC3?
There's may be some configuration of MMC3 that might work, but I'm not an expert, and you wouldn't be able to use the features of the mapper like scanline counters, etc.
Mapper-specific advice:
- NROM-128: Be careful not to write to $8000-$FFFF. Test with a breakpoint on writes to ROM.
- NROM, CNROM: Be careful not to overwrite CHR ROM. Test with a breakpoint on writes to PPU $0000-$1FFF.
- NROM, CNROM, ANROM, BNROM: $FFD0-$FFF9 of each 32K PRG ROM bank must be unused.
- UNROM (2): $FFD0-$FFF9 must be unused.
- UNROM (180): $BFD0-$BFF9 must be unused.
- A53: Write to register $81 only once, at the beginning of the program, and match the values written to $80 to the size of the entry: $00-$0F for 32K entries and $10-$1F for 64K entries. Specify whether $FFD0-$FFF9 or $BFD0-$BFF9 is unused.
I'd recommend asking on the nesdev discord though. Pino is a good person to ask.