I don't have an 8 slot G5 here, so I am going to make some guesses.
Dimm0 is the topmost slot (marked 4, highest slot on the logic board),
OR (guess 2):
Dimm0 is the top center slot (marked 1 on the upper bank)
That makes Dimm3 as either slot 1, or the second guess, is slot 4 (still in the upper bank of 4)
So, how to proceed? Here's what I would do:
Do NOT move any RAM sticks, except for upper bank slot 1 & 4. Before you swap any sticks, reseat ALL the sticks, but be sure to place each stick back in the same slots. AND - test your system. I would look at each stick, to make sure that each pair is the same manufacturer. If the sticks are all the same capacity, then perhaps they were all bought together, and are already same manufacturer.
Pairs of RAM (same capacity, same manufacturer) should be installed in pairs, working out from the center slots: Slots 1 are a pair, slots 2 are a pair, etc. DO NOT move the sticks unless you need to - keep track of any changes that you make. Just for troubleshooting, don't make a change in stick slot, unless you first try moving those 2 (swap upper bank slot 1 & 4) that I mentioned first. One of those will be the Dimm3/J14
Most important, is to reseat (without moving the sticks between slots), and test your PowerMac to see if that's all you need to do.
If that doesn't help, THEN try to swap the upper bank slots 1 & 4. and test again.
If you STILL get boot errors, or the memory is not recognized properly, come back with the error.