On ali aladdin v there are reports of up to 1.5gb of ram working. 3gb may be possible but I haven’t seen anyone do it yet.
On mvp3 (what OP has) I have only seen up to 768mb of memory installed. Mvp3 has a less forgiving memory controller.
Personally, I am running 640mb on a mvp3 right now. (I would be running 768 if I had another stick)
Believe it or not, this is actually 128mb per side bga pc150 sdram yes you read that right lol
Cachable memory isn’t much consern when running a k6-3/2+/3+ cpu. Though the less memory you use the higher your benchmarks will get as long as you don’t run out of memory that is. Someone with better understanding of memory controllers may be able to explain why that is, but my understanding is that all that ram takes longer to keep refreshed or something rather. And it’s different than chachable area.