Just to add a bit to what has already been correctly quoted.
There are two sets of regulations out there, the first set are according to the The Examination and Testing of Miniature Steam Boilers (Revised Edition 2008), better known as the "Blue Book"
distributed by:
Midlands Federation of model Engineering Societies
Northern Association of Model Engineers
The 7 ¼” Gauge Society Ltd.
Southern Federation of Model Engineering Societies
and the second set of rules are the rules as laid down by the Model Power Boat Association (MPBA).
The first set of rules do not require pressure tests for manufactured boilers below the three bar-litre limit but do recommend annual steam tests and the second set of rules does require pressure tests of all boilers. To ensure you can run your model everywhere you would need a current (Dated) numbered pressure test certificate but if you were only going to run it at a club that used the “Blue Book” you would only need a steam test certificate.
As for the 3 bar-litre rule your 3 ½ inch Macsteam boiler has a working pressure (design pressure, not safety valve lift pressure) of 4 bar and a capacity of 750ml so that puts it slap bang on 3 bar-litres. If you want to avoid the pressure test requirements I would ensure that the capacity is measured as 740 ml.
Assuming the “Blue Book” is being used the safety valve can be set at any pressure below 4 bar but during the steam test the pressure must not rise above 10% of the working pressure with the flame on full and the safety valve fully lifted.
The pressure gauge must also be checked against a calibrated and certificated test gauge.