Stu covers some good points to consider.
With practicality in mind Cambridge_Pulsar, if you use any thickness of spacer and you can still tighten the wheel nuts by at least 8 full rotations, you should be safe against the wheels falling off your car.
With the spacers themselves, some of the universal ones don't always sit correctly, so getting ones that are specific for the stud pattern keeps them properly alligned with the studs and centred on the hub.
If you use spacers that push the wheel out too much from the hub, like Stu said, you get the problem of not enough wheel stud showing to properly tighten the wheel nuts. In this case, you need longer wheel studs. But as well as this, the wheels will no longer be properly centred on the hub once you space them out too far, they will locate around the wheel studs only. This could cause vibrations and stress on the wheel studs, in which case the soloution is to get hubcentric wheel spacers which keeps everything located properly.
Spacing by 3mm should avoid this though. Of course, the ideal soloution would be to get properly machined mounting bells, assuming your disc setup is 2 piece.