Not sure I understand your last sentence about the blade sitting back 3mm.
Anyway, before you start messing around with washers and shims, sort out the tracking. Remove all the blade guides and supports, fit the blade and spin the wheels by hand. It doesn't matter where on the wheels the blade sits (within reason) – in any case that will change with different widths of blade. What matters is that it stays in the same place as you spin the wheels, and doesn't work its way forward or back and come off.
If the blade comes off the wheel, adjust the tracking by turning the knurled knob on the top wheel centre. If the machine has been dropped or damaged so as to twist the frame then you may have to think about either 'untwisting' it, or compensating by using shims or washers on the wheel mounting components.
Once you have the blade tracking ok, then adjust the table so it is at 90° to the blade – the left/right adjustment is easy as, but forward/backward you may have to shim the table mounting points.
Once you've done the above, you can re-fit the blade guides and supports. These must not be used to actually position the blade, just to support it when making a cut. Nip up the side guides using a bit of newspaper between the guide and the blade To give a couple of thou clearance. Adjust the carbide tipped rod that provides support to the back of the blade so it is a mm or two away. This gap will close up when you take a cut, but return when you take the pressure off.
If you do actually discover that the bandsaw frame is twisted out of shape you might need a bit more help – it is sheet steel construction so potentially easier to get back into shape compared with something that has a cast iron frame, but it might involve a fair bit of dismantling.
Also, where are you located? It might help in case you need to look at someone else's machine or make some comparative measurements.