Hydronic heat radiators not used for long time - should they be drained?
It is better to keep a hot water hydronic heating system full of water all the time. This keeps air out, and prevents the inside of the radiators from rusting (assuming these are traditional cast iron rads). Recirculating the same water also helps limit the introduction of minerals from your water supply, so your rads and boiler won't clog up with mineral deposits. For these reasons, you shouldn't empty and re-fill your system more often than needed.

The exception is if your home will experience very cold temperatures, and the water could freeze. In this case you definitely want to drain the system to avoid having ice form and crack the rads.

If your system has had water in it for the last 3 years, I would not drain it unless you have reason to do so when repairing/replacing your boiler. Just bleed your radiators to ensure they are full of water. If the system has been empty, you may want to flush some water through to rinse out some sediment before reconnecting it to your boiler.