I just came into an A3000 as well, the 2MB chip is already populated. 25MHz 68030+68882 unit, and it also has the switchable 1.3 vs. 2.0 selection screen (?!)
It has a weird fast-RAM configuration of 2.5MB - the DIP sockets near CPU Accelerator slot are all populated (8x KM44C258AP-8 DIP RAM, for a total of 1MB) and then 1.5MB (?!) of ZIP memory? Or perhaps it is 2MB of ZIP memory and 512KB of it is being eaten by a mapped Kickstart? Not sure yet, first rodeo with an A3000.
I know that the 1st ZIP bank can't be used while the DIPs are populated, and there seems to be 16 zip chips installed, which is 2 banks (groups of 8). Thus, i'm assuming I have 256k×4 zips installed, and these 2 banks = 2MB.
Seems if i stick to existing zips, i can max out at 3MB (of zips) since I can only populate one more bank, and it has to match with same type of RAM
I am hoping to have an accelerator in here at some point, which will have much faster onboard memory, so perhaps it isn't worth looking into expanding the onboard memory at all (but I can't resist...)