EU is already increasing military spending, but it doesn't need to be anywhere near 3x - unless the US actually follows through on its threat to invade EU nations of course, and then we're into WW3. Everyone loses.
The EU is funding Ukraine more than the US is, and Russia is having problems with just the Ukraine. They have nukes, sure, some of them may even be operational, but conventionally they'd be no match for NATO-sans-US, even right now.
Currently the EU is spending a fucking fortune paying for US weapons - the real reason Trump wanted NATO to spend more is because it's all just been income to the USA. Fuck that. It may take a while to wean off the US teat, but its now inevitable I think. Americans have demonstrated the inherent weakness of their political system, and the world has realised it cannot count on the USA to be a reliable or predictable partner nation.
Over time, the US will inevitably lose its bases in Europe and its ability to project power will be curtailed, it will lose a large chunk of its economy of scale with weapons when coutries aren't buying them (and therefore funding their R&D) and hopefully that reduction in US world power will change things for the better. At this point, even China is looking like a better partner-nation than the USA to Europeans. At least they do as they say, and they're stable.