Spitting three balls out just after loading them is wrong. It is supposed to load three balls under the playfield: one in the right cannon VUK, one in the left cannon VUK , and the last one in the normal left VUK. If this is not happening, it will try spitting more balls out. I suspect a dodgy opto in the VUKs, or a subway diverter that is not working correctly.

If anything is wrong on this machine, its usually 90% found in balls not getting where they need to be, or opto-related.

Try to operate the diverters in the coil test first (not sure if they have both hold and high power coils). If the coils work fine, first clean the opto's. If the problem persists, maybe reflow the solder on the opto's in the vuks.

A tip I've recently read here: leave the machine on in switch test, and go do something else while in the neighbourhood. If you hear a switch sound every now and then, it could be that dodgy opto.