Yeah - I agree, that it's possible to learn all that in 3 months, if you don't work full time, but it will be far from being professional full-stack dev who need enormous amount of knowledge not only about chosen stack but also about security, optimization, architecture, good practices, etc.
I work with front-end about 10 years, and i can write rest api + mongodb, but it doesn't make me a full-stack - I'm far from that :)
So. Can one learn full-stack basics (or even more than that) in 3 months? Sure, under some circumstances. Can one be a professional/good full-stack developer in 3 months? IMHO only if one is a genius :)
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