Using relay contacts in series to switch DC loads
Using relays in series only increases the resistance of the relay when open.
It can also serve as a safety feature, would one of the relays fail.

It does not increase the switching capability since:
- you cannot guarantee they open at the same time.
- the rating is based on the short contact resistance increase during disconnect. This causes heat (ohms law) and will weld/damage the contacts upon close/opening action.

Your only option is to buy a relay rated for 3A 220VDC. Keep in mind that you need to follow the derating specifications when your load is not a pure resistor.