I assume you meant 3000 milisieverts (3 sieverts) or even 3000 sieverts. 3000 microsieverts (3 milisieverts) is most definitely not fatal, as it is similar to

I assume you meant 3000 milisieverts (3 sieverts) or even 3000 sieverts. 3000 microsieverts (3 milisieverts) is most definitely not fatal, as it is similar to the yearly exposure.