Godmorgon, 'MalmoV40'! I have no experience of the V40, but it is most unusual for any battery to fail after only three years unless it was faulty - is yours still covered by guarantee? If yes, I would take the matter up with the dealer.
If not, and you are on your own, a long, slow trickle charge may revive the battery - avoid 'fast' charging, which can further damage a failing battery.
If you are successful in reviving the battery, you need to find out why it went flat in the first place to avoid having a repeat performance! I find the solar panel trickle chargers that connect to the battery with 'crocodile' clips useful for maintaining a charge if you are not using the car very much. I don't know if these are available in Sweden, but are often obtainable from Aldi in the UK.
Finally, it is not recommended to run any car without a battery in circuit, so it is probably not a good idea to start on one battery and then fit another. Wouldn't a set of good quality jump leads overcome this difficulty?
I'm sure others with specific knowledge of the V40 will give you more informed answers. The above is only a generalisation based on many years of car ownership.
MVH, John.
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